Of course you can, but your AT&T phone will be working at 2G speeds. If you're fine with that, go right ahead and do it. I just don't see the point of paying a lot of money for a fantastic smartphone and then handcuffing it with your network.
Originally Posted by brettschulte
Any GSM MVNO will let you use a T-Mobile or AT&T phone since both are GSM networks. Once again, the Lumia 800 uses AT&T's 3G band (this is different than GSM/Edge/2G). Straight Talk uses T-Mobile's 3G bands. You'll be able to talk, text, etc., but your data speeds will be Edge/2G instead of 3G or 4G.