Anyway, if I was in the US, I'd likely be with ATT, not necessarily because they have the iPhone, but because they're also using the global standard GSM/UTMS. Anything else 3G and earlier is just barbaric. Every carrier in Canada (there are 3 national ones, two sharing new resources) now has GSM/UTMS. I think the US companies had better get on the ball and standardize when they move forward to 4G.
I was originally with T-Mobile and more recently AT&T for precisely this reason, so that my phone would roam properly when I went to Europe.
Of my various European adventures, though, I've found that I don't actually use my phone much because it's so shockingly expensive ($1.29/minute, I believe), versus just using Skype or whatnot. Maybe I could ditch the phone altogether, but certainly I can't discount the need for a phone as an emergency device.
The 4G LTE standard (
Wikipedia link) will be supported by both AT&T and Verizon and is clearly intended to be the end-all-be-all of wireless transmission standards. An interesting question is whether they'll be able to make phones that can operate world-wide, given the wide variety of different frequency bands.