You would need a GSM provider such as Cingular or Tmobile, since GSM is all there is in Europe. There are also other smaller regional providers scattered around the country that use GSM. Tmo isn't in my area - we have Cingular & SunCom.

If you're planning on using your phone overseas, I would recommend that you either purchase an unlocked phone or see if your service provider will provide unlock codes (or pay a company online to unlock the phone you get from the service provider). With an unlocked phone, you could purchase a prepaid account with a european cellular provider & just put their sim chip in your phone. This would cost much less than paying Cingular or Tmobile's international roaming rates which are at least (if not more than) $2/min.