AT&T recently announced that it was changing up its data plans for all smartphones—including the iPhone. Instead of a $30 unlimited 3G plan, there's now a 2GB monthly cap for $25 per month, as well as a $15 tier that gets you 200MB of data. Though this upset some heavy data users, we here at Ars argued that most people should be able to save at least a little bit of cash each month, as 2GB can be a difficult to hit for many of us.

A number of our readers pointed out, however, that there are several factors that can contribute to higher data usage—factors that would be amplified by the release of iOS 4.0 and the new iPhone 4 along with it. With the new OS, there are some new options to take into consideration when the OS lands on June 21, so if you're right on the edge of your data plan on the iPhone, here are five reasons why your data usage might go up:

More streaming music thanks to multitasking

Reading the comments on our previous two stories about AT&T, it seemed like there were two camps: people who streamed music all day and people who didn't. Thanks to the multitasking capabilities added to iOS 4, we'll all be able to stream audio more easily by keeping Pandora, Last.fm, NPR, or our other favorite streaming apps alive in the background while we do other tasks. Depending on how long you stream the music, you may end up coming closer your 2GB monthly limit faster than you did before.

Netflix streams on the iPhone

Netflix recently announced that it plans to introduce an iPhone app this summer, and CEO Reed Hastings demoed it during the 2010 WWDC keynote. The app will stream Watch Instantly videos over both WiFi and 3G and, since it's one of the most popular online video services, it seems like a given that Netflix subscribers will probably start using more data because of it. Hastings said that the app makes use of HTTP adaptive bitrate streaming, so the videos won't suck as much bandwidth while you're streaming over 3G. Still, with a few thousand more TV shows and movies available to you on your phone, you may bump up against that 2GB limit a little quicker than expected.

HD video uploads from the iPhone 4

The iPhone 3GS can already take videos and upload them to YouTube and MobileMe, but the new iPhone 4 can take those videos at a pixel-dense 720p. Not only that, but users can craft iMovie creations through Apple's new app, giving people an even bigger motivation to put their kid, vacation, police beating, and skateboarding dog videos online straight from the phone. Bigger and more elaborate files translates to more bandwidth usage.

Bigger image uploads

Similar to the HD video uploads, iPhone 4 will have a higher resolution camera for taking still images. (In fact, it will have two cameras!) Those larger image files will require more bits sent to places like Twitter, Flickr, your mom, or all of the above. And, with such an improved camera sensor, we have no doubt that users will start sending more pictures than ever after the new iPhone lands.

iAds

Sure, there are plenty of iPhone apps that already have ads, but how many of them are meticulously crafted using all the latest HTML5 techniques like the ones Apple demoed at WWDC? With the capability to do 360 degree views, games, quizzes, and videos, the potential is there for Apple's new iAd system to use noticeably more data than the current selection of mobile ads. And, since the companies involved have such big ad budgets, you can bet that many iPhone developers will be quick to start adopting iAds in their own apps.

What else?

These were the top five things the Ars staff could come up with, but you undoubtedly have plenty of other reasons why your data usage might go up after the iOS 4/iPhone 4 launch. What do you plan to do with the new software that might start sucking up even more bandwidth? Are there any ways you think the new hardware or software might actually cut down on your data usage? Let us know in the comments.