can you use the same iPhone in USA and Ireland

I have the old iphone and my contract ends dec 09. However I am moving to Ireland for 1 year to get my masters. If i upgrade here in the USA to the new phone (i have to pay for service anyway) can I replace the sim card in Ireland and use the phone there (assuming I dont have to get a 2 year contract since i am not buying a phone there)? That way i only have to buy one phone and just switch the sim cards depending on where I am.

Thanks for your help.

No, the iPhone is locked to AT&T, and unless you unlock it you can't(and right now there isn't a way to do it yet) . What you can do is buy the Abroad rate plan for Ireland and then use it, but it will get VERY VEERY expensive. What I suggest you do is either wait to get the iphone until you get back, OR get the iphone and take it with you to Ireland, but turn it to airplane mode and then turn wifi on. And get a disposable/pay as you go phone in Ireland and set your iPhone to forward to that new phone. OR another thing you can do, which is probably the best thing to do is get your iphone, take it with you and then turn it to airplane mode, and then turn wifi on, and then download the VoIP App that is in the App store. But of course you have to be in a wifi area to be able to make/receive calls but these days, especially if you are on a college campus it shouldn't be a problem.

What I do when I go to Europe on Vacation, which is quite frequent lately, is get the pay as you go phone and set my other phone to forward to the new number.

Oh well I thought it was stretch. Maybe when I get back to the states there will be a 3rd gen iPhone. Thanks for the response.