I was just talking to some AT&T reps about that yesterday when I was (unsuccessfully) trying to get them to give me a Captivate early...
I've had my unlimited medianet plan ($20 for unlimited data/text/mms) for the past six and a half years. When I picked it up, they was no separate plan for smartphones, and the rep told me that as long as I didn't cancel it, I'd be allowed to keep it. About two years after that, I picked up a Treo 650. No problems with it. And about another two years after that, I upgraded to a Blackjack II. Again, no problems. While I don't use a ton of data on a regular basis, once or twice a year I hit 3~4gb over the course of a week or two.
Anyway, when the ATT rep opened my account info, they said I'd have to upgrade because my plan doesn't support smartphones. At which point I pulled my BJII out and set it on the counter. That royally confused the rep... he kept saying 'but that shouldn't work!' So he pulled the manager over, and after a bunch of hemming and hawing, they told me to come back on Sunday. They'll go ahead and try to enter the phone upgrade into the computer. If it accepts it, then I'm all good. If it doesn't, they can cancel the upgrade and I'll still have my old plan. But he's kinda suspecting that it'll work since I've had a smartphone for the past 4 and a half years with no problems.
Then the manager dropped this little tidbit: AT&T checks the IMEI number of the phone, the data plan on your account, and their database of smartphones. If the IMEI number is in their database of smartphones, you must have a smartphone data plan. BUT... the only phones that are in their database of smartphones are the ones AT&T carries. So I can pick up a Nexus One or a Canadian Galaxy S, stick my SIM into it, and everything will work just fine.