If you do much international travelling, stick with a quad band phone from AT&T. Their GSM phones are compatible most everywhere except some countries in the far east. The international roaming works great - I've used it all over Europe and a couple countries in S. America. You can add and drop it at any time, altho' AT&T usually wants you to wait a couple months before you drop it to be sure all charges have come in from the various countries.

I'm not an Apple fan, so I can't speak to the iphone - good or bad; tho' I can't imagine that you'll be able to run winmobile apps on it...

Problem with AT&T(one of them anyway) is that they change their product line more often than most people change socks. Check out their offerings from HTC. They used to offer the HTC Wizard which got great reviews, but it's been dropped recently as AT&T goes to smaller smart phones.

phonescoop.com is a pretty good website for comparing the phones offered by various carriers.