I have AT&T and want to stick with them but i love the my touch phone and its T mobile! And i know of other phones you can switch the network on it even though its by a different phone company, and i was hoping that this is one of those phones!! [:
yeah, you can unlock the G1 so i'm pretty sure it's already been discovered how to unlock the mytouch 3g. i'm assuming you don't know how to so your best bet would be to buy an already unlocked mytouch, or learn how to do it yourself (shouldn't be too hard, unless you don't know much about computers, files, and whatnot)
but yes, long story short you should have no problem using AT&T on a mytouch