Yes *if* they have the right bands and international roaming is enabled.
Most US phones are locked to the US carrier, and have to be unlocked to be
used with a non-US SIM.  See the FAQ below for more info.

I have a Motorola V220.  It worked fine on International roaming in the
UK last Winter.  Before I went back this past fall, I took the phone to
a local mom and pop outfit and had it unlocked (Cingular refused to
help with this.)  When I got to the UK I found an O2 shop and got a sim
card and a pay as you go plan.  The sim card was about five pounds, or
a little less than ten bucks.  Worked great and gave me a UK phone
number for my business trip.

I got my Cingular RazrV3 unlocked for the purpose of using it in Europe.  I know I can now just go into the corner store and get a prepaid sim card on Orange or Primus or one of those.  My question is whether I will be able to use the GPRS in Europe with one of those cards to surf as I can use webmail to do my email stuff ...what's the use for that in central Europe (France, Holland, Belgium, Luxembourg, Germany) area?