Can I get the Sidekick ID from TMobile and have an AT&T plan?

I really want the Sidekick ID from Tmobile. However, I have the AT&T plan and I want to stick with AT&T. I've heard that a TMobile phone uses a SIM card like AT&T does; however, the sidekick ID has the MyFaves plan when AT&T doesn't. Also, I want the SideKick ID as a prepay for AT&T. Is this possible? Or am I getting my hopes up?

I dont think so i asked a question like this
and the sim card wouldnt work?