I use AT&T, my family uses AT&T, my gf uses AT&T...it doesn't make sense for me to switch. That said, I've been wanting an Android phone well before T-mobile even came out with the G1. I had just gotten a blackberry pearl though, so I didn't get the G1. I was very much considering getting another carrier's Android phone this summer and using on AT&T's edge network, but now AT&T is going to have some Android phones of their own I'll get one of those and have 3G.
I am most certainly going to be getting HTC's Lancaster.