ok a few things. First to answer OP's question is yes, it is possible with any unlocked phone. problem is that AT&T has certain IMEI numbers that are recognized as smart phone IMEI's. Like my friend had an HD2 that he brought to AT&T and they recognized it and immediately changed his data plan to a smartphone data plan. Although there are other people with smartphones who have the AT&T system tricked because the system doesnt know what their IMEI number is for.
So yes, it is possible. And yes, as for the NON-Smartphone $50 unlimited talk, txt, and data plan, yes it does exist. AT&T just started it recently and no its not for just international. Its directed towards NON-Smartphones. But again, if they cant recognize your smartphone IMEI, then they wont know.
As for the $50 sim card, no there's no such thing as a $50 sim card and no, its not "just for international phones" haha. Sim cards are $5.95 and if a store tells you anything higher, then call AT&T sales. Because they are ALWAYS $5.95 over the phone.
BOOM! glad to help haha
That aside, I've been toying with this concept myself. I've been looking everywhere for good candidates for a smartphone that ATT doesn't sell and that is compatible with their HSPA network.
I have a couple unlocked T-Mobile phones that the data works on, but only at EDGE speeds because they don't have the proper frequencies to support ATT 3/3.5/4G.
Does anyone have any new suggestions?
EDIT: So far I've found that the HTC Wildfire S A510a, and the HTC Sensation Z710a are both compatible with ATT's HSDPA 850 and GSM 850 bands and may be good candidates. I already have a T-Mobile HTC Sensation Z710e and really don't want to buy another phone that's practically the same .