Doesn't matter if you are already with ATT.
No matter who you are with you buy the phone and then when you get home you use iTunes to activate it.
http://www.apple.com/iphone/usingiphone/activation.html
Apple/ATT says that you can not use the iPhone as a modem. Kind of sucks but then from what I've read the EDGE network is so slow that you wouldn't want to anyway. WiFi is everywhere where I live so no big deal for me but it would be nice to have decent internet speeds when I was someplace that I couldn't log into a WiF network.
The online Apple store say two weeks delivery but like Simon I'm going to wait for version 2 of the phone unless my current phone dies on my first.