I'd be willing to be that the cheep plan is much like AT&T's cheep plan, only 2G (or 2.5G). But if one has a 3G enabled phone on AT&T and tries to use 3G on a 2G plan, it only gives you 2G.

Every 3G phone AT&T and TM sell they require you to get the 3G package if they're subsidizing it. But they can and will block 3G access if you just drop the sim in that's linked to a 2G plan. Otherwise, why would anyone buy the more expensive plan?

Personally, I have the cheep plan from AT&T as well, and don't mind it, since most of the places I normal go have wifi. And since my phone technically can't do 3G on their network, they have no way to force me to update to the better plan.