This post has been updated since it was first published. Please note the updates at the end of the post.

While we had no problem getting an iPhone 4S while keeping our grandfathered data plan, that’s because we ordered direct from Apple. On the other hand, we’ve received at least two reports of readers who are being told by AT&T that they have to give up their unlimited data if they want an iPhone 4S. Uh oh. Luckily, there’s a way to make sure you keep your unlimited plan: don’t give att.com the time of day.

It’s a really shady move on the part of AT&T, who has been boning things up all morning. Here’s the deal.

If you go to Apple.com’s iPhone 4S pre-order page and sign up for a new handset through AT&T, Apple’s site will register that you have an unlimited data plan and everything’s hunky dory.

Whatever you do, though, don’t order an iPhone 4S directly from AT&T. When you try to order one, the AT&T website will tell you:

Your new device requires a different Data Add-on Plan than you currently have. We selected a compatible Data Add-on Plan for you. If you want a different plan, you can change the Data Add-on Plan by selecting Change here or from within myAT&T after you receive and activate your new device.

If you notice and try to change the data plan manually, AT&T won’t give you the unlimited data plan option (unlike if you do it on the Apple website.)

So basically, here’s what it boils down to, at least as far as we can tell. If you order an iPhone 4S from AT&T, they will force you to give up your unlimited data plan. So don’t even bother with att.com. Just head on over to Apple.

Hat tip to Chris and Teng!

Update: Reader Michael explains that what is going on isn’t nearly as devious as it appears, just confusing.

I believe this may be a misunderstanding. I have the grandfathered $30 unlimited iPhone data plan on my current 3GS, and my purchase on the AT&T site looked just like the example here. It preselected the $30 plan for my new 4S and despite the same ominous message, I checked out with the $30 “iPhone data plan”. I also ordered a new 4S for someone else right afterward. That order required me to pick from one of the new data plans ($25 or $45) with radio buttons before I could proceed. That order summary showed a new ongoing monthly $25 fee for the DataPro 2GB plan. It did not offer anything called a “$30 data plan for iPhone.” Thus I believe that my order and yours are in fact grandfathering the old, unlimited, $30 plan, and the message is just misleading.

In other words, if you ignore this message, you still get your unlimited data plan. Whew. Here’s the question, though: is AT&T trying to trick people out of their unlimited data plans? Sure seems like it.