[Rick#2]

Maybe this is a dumb question, but is your AT&T SIM a valid one, ie: prepaid or otherwise still in use? If you're trying to use a SIM that doesn't have an active account linked to it, it certainly won't work.

I don't see why a different SIM couldn't work in your unlocked Sprint phone. If you have a prepaid AT&T SIM card, for example, you ought to be able to pop it in and for all intents and purposes, you aren't on the Sprint network anymore and are just using AT&T services with your prepaid account.

No different than taking the phone international and popping in a prepaid European SIM. There have been reports of Telus users (ie: Canadian) using prepaid American SIMs down in the States without any issues.