Is an ATT Iphone 3GS T-mobile compatible?

Anybody know about ATT Iphone compatibility with Straight talk by use of a SIM card?
I'm not much of a techie and don't know where else to look.
The Straight talk website is not helpful. Though I'm aware the phone would need to meet these requirements:
"T-MOBILE COMPATIBLE GSM PHONE, OR AN UNLOCKED GSM PHONE
make sure it operates on our compatible frequencies" (our being straight talk)

"SIM card will only work with an AT&T or T-Mobile compatible or unlocked GSM phone"