I wrote recently that the new features in Apple’s iPhone 4 have me wondering if I should finally get a smartphone. But for me, the technical aspects of the iPhone are actually the least important part of a decision like this. The iPhone has fundamentally changed the way people communicate and interact. And the changes aren’t always positive. I’ve been thinking a lot about what it would mean to bring an iPhone into the house.

Here’s the situation. I’m not an IT guy, but I am a technophile. I like cool technology, and I love well-designed products.

But in our house, all computer activity stops at 8:30PM. Without that rule, we talk less and sleep worse. What would happen if we brought smartphones into the house? I’m not sure, but I feel my blood pressure rise when I read about people (including many TMO writers and readers) using them to check news headlines in bed or fact-check conversations on the fly. This behavior wasn’t the original reason I stayed away from smartphones, but it’s proven to be the most durable.

The thing is, as phones get more compelling, it gets harder to stay away. Peer pressure is one reason for some (take TMO reader Geoduck’s comment as an example); two others are money and convenience. For many of us, it’s now cheaper and easier to own one smartphone than to lug around a phone, camera, camcorder, and GPS.

These mobile devices will only become more a part of our lives. So it’s time for some discussion about their appropriate role. People write about gadget etiquette all the time, including here at TMO. But I feel we’re at a transition point with iPhone 4. The sheen has worn off. These devices are part of modern life. The iPhone and it’s competitors (what David Pogue calls “app phones”) are all grown up. It’s time for us to grow up, too. Let’s talk about what it means to be an adult with a smartphone.

Basic Principle #1: The Message You Send Sends a Message

Apple stresses that the iPhone connects people with one another. That’s true, if the people aren’t already physically proximate. But what about the people standing next to you? You’re ignoring the world around you with every email you read, every text you send, and every check of facebook or twitter or — indeed — The Mac Observer. Let me repeat that: if you’re communicating with someone who isn’t present, then you’re not communicating with the people who are present.

The result is that the mere sight of a mobile device sends a message to the people around you. If you who wear your phone on a hip holster or always wear a bluetooth earpiece, you tell everyone around you that you are fully prepared to stop paying attention to them at any time.

And when that text, email, or call does arrive, or when you glance down to check on that push notification, everyone around you gets the message that they’re less important to you than the people at the other end of your phone.

This leads me to Rule #1. Mobile devices should remain invisible. Not necessarily hidden (although that’s fine), but invisible. If you’re using your phone in such a way that others notice it, then you’re being rude.

How do you follow Rule #1?

Minimize the visibility of your phone and peripherals. Turn off audible alerts.

Never use your phone during a meeting or conversation. You’re not being sneaky, you’re being an ass. If you run a meeting, institute a no-phones policy.

If you do need to check your phone, excuse yourself from the meeting or conversation first. (You don’t need to say anything, just move away.)

Don’t fact-check the conversation unless someone else specifically requests it. If you’re really itching to look something up, ask first: “Should I Google it?” If no one answers, the answer is no.

If you want to use your phone for something social (to show pictures, to share music, to take a picture or video), ask first: “Our new dog is really cute. Want to see her?” (Again, if no one answers, the answer is no!)

Being on your own — walking down the street, say — does not exempt you from this Rule. If you’re playing with a phone, anything that goes wrong (bumping into people, getting hit by a car, falling down a manhole) is your fault.

Basic Principle #2: Focus is Binary

There are plenty of words being written about how always-on, multi-tasking behavior damages critical thinking. More importantly, the concept multi-tasking is basically a lie. Working on multiple tasks degrades our ability to do any of them well. There’s cognitive psychology research being done to quantify the effect.

But come on, we don’t need doctors to tell us that we can only do one thing at a time. Have you ever had a stimulating conversation with one person while you write a brilliant letter to someone else? Of course not.

The result is that constantly running to your phone doesn’t just make you emotionally distant, it also makes you dull and unpleasant. For example: Joe is with his friends Adam and Sarah when he receives a text from Jane, who has suggested a time and place to meet later that afternoon. He looks up from his phone and says, “That was Jane. I’ll have to leave after lunch.” Maybe Adam was hoping to get a ride from Joe after lunch, so this information is relevant to him. But Adam was in the middle of talking to Sarah about something completely different, so Joe’s comment is annoying and distracting. It comes across more as if Joe is forcing Adam to make a decision immediately, on Joe’s schedule, about whether he wants a ride.

Hence we require Rule #2. Do one thing at a time. There’s a small door between the analog world around you and the digital world in your phone. Respect that door, and don’t try to be on both sides of it at once. You’ll be more likely to say something interesting in an analog conversation, and you’ll be more likely to get something from your digital excursion.

Rule #2 is somewhat Zen. Some tips for putting it in practice:

Follow Rule #1. In addition to making you more pleasant to be around, it raises the barrier to stepping between the digital and analog worlds.

Put the analog world first. You are at your most effective in the analog world, because all your senses and faculties are fully at your disposal. If you’re going to do or say something worthwhile, you have your best chance of it happening with the people right in front of you.

If you’re bored, don’t start fiddling with your phone. Instead, look for one thing to think about in depth. You won’t be bored for long, and you also won’t be so boring.

When you return from the digital world, return quietly. Pause to dump your brain’s working memory. Even if you learned or did something interesting on your digital adventure, no one else went with you. You walked out on the conversation. You are now obliged to re-integrate yourself into the conversation as much as if you had walked out of the room.

Basic Principle #3: Behavior and Expectations are in Positive Feedback

People express disbelief when I say we shut down our computers at 8:30PM. How would anyone get ahold of us if there’s a problem? How do we get all of our work done? The disbelief comes from an expectation of how and when people are available. Expectations about availability — on phone, email, social networks, or Twitter — are understood to drive our behavior.

But behavior also drives expectations. How do we keep our rule? We simply plan to be done by 8:30PM with anything that requires a computer. Very occasionally we’re overloaded and fail to meet the deadline, so our sleeping habits suffer accordingly. But it’s rarely a problem. Much more common is that one of us “has to do one quick thing” — send an email, check a website — for something that no one will use before the following morning.

This behavior is fascinating to me. Why do we feel like there might be “something important” that we don’t want to miss? When you stop and think about it, even the important things usually don’t have an immediate deadline. You can answer an important email tomorrow morning. In fact, you might be able to answer more constructively if you’ve had a night’s sleep. Or if you wait to deal with it over a phone call or in person.

Hence, Rule #3. Reserve urgency. Most of what we do is not that important. Behave accordingly. If you’re not always in panic mode, people will take you seriously if pull out your phone to get something done. (Remember Rule #1, and excuse yourself first.)

I’ve found I don’t need a bullet list to follow Rule #3. I just need to lower my opinion of myself. In fact, we’d probably all benefit from occasionally reminding ourselves: “The world will go on without me.” Believe that, and you’ll feel a lot less pressure to check in on FourSquare. Or update your FaceBook status. Or respond to comments on your blog.

Feel the weight lifting from your shoulders? Great. Now put down your phone, turn to the person next to you, and try out a conversation that’s carried on by something other than your thumbs. You’ll thank me when your companion notices she has your undivided attention, and smiles.