Doubtful. I have a Nokia e72 and N900 which is currently working without issue on the unlimited $15/MediaNet plan. This is also the case when I use my AT&T SIM card in my T-Mobile Motorola Defy (works on AT&T's 850 3G band), as AT&T does not have a flag on the IMEI number.
However as soon as I put the AT&T SIM back in my AT&T branded Motorla Q, I immediately received a text from AT&T that my plan had changed to the 2GB "smartphone" plan.