It will work with AT&T SIM. The phones are unlocked.
My biggest question, like you, does MMS, more specifically, incoming MMS? The consensus is unclear.
I ported my number out of H2O and to Net10. I want to get this phone but I bought the 928 a few weeks before release. The only carrier I personally tested that everything works, MMS, SMS, Data is on Net10. I wanted to go to AT&T but couldn't get a solid answer if MMS does work. It might be a show stopper to some but for me I just ported. Been meaning to leave H2O but will miss certain features. But not supporting Google Voice with conditional call forward was real reason I wanted to leave.
If anyone has an AT&T SIM and can confirm everything is supported, I'd pick this phone up in near future.