To use GSM in Western Europe you'll need a phone that will be able to
operate on 900 and/or 1800 Mhz.  North American GSM is either 1900 Mhz
or 850 Mhz.  Depending on who your US GSM carrier is you'll need
either 1900 or 850 or both.  T-Mobile uses 1900 only.  ATTWS and
cingular use both 1900 and 850 though cingular uses more 850 than 1900
in their system.  ATTWS uses 1900 throughout their system and also
uses 850 where they have the licenses to use that.  Some phones have
all four bands such as the latest Handspring Treo, the NEC 515/525,
Motorola V600.  Others have triband phones that either work
900/1800/1900 or 850/1800/1900.  900/1800 is preferred for Europe, but
you could theoretically get by with 1800 only in Europe as well though
you won't have as many networks to choose from.

You can buy a prepaid SIM when you get to Europe and this is very
practical if you're going to be spending a good number of days in one
country.  Many of the European prepaid "pay as you go" accounts do
allow roaming, but it is quite expensive.  Don't forget that European
mobile accounts all incoming calls are free, but when someone calls
your phone it can be quite pricey as the caller pays for the airtime
unlike in North America where the called party pays both incoming and
outgoing.

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          remove NO from .NOcom to reply

Bob,
yes I would just get a Prepaid Sim Card from a European Provider once
you get there. What countries are you going to be traveling in? Get a
Sim Card in the country you are going to staying in most. Your best
bet would be to get a Vodafone Sim Card. I don't know what country you
are going to be buying this Sim Card but here are Vodafone Networks in
Western Europe:
Germany: Vodafone D2
Switzerland: Swisscom Mobile
France: SFR-Vodafone
Italy: Vodafone
Spain: Airtel-Vodafone
England: Vodafone
I travelled to all those countries and Vodafone provides excellent
service in most areas of those countries. I have several European Sim
Cards( Vodafone D2 in Germany, Swisscom Mobile in Switzerland,
Airtel-Vodafone in Spain, also an E-Plus Sim Card in Germany) and I'm
very pleased with Vodafone's service and coverage.
You can use a GSM purchased in the U.S., if it is a tri-band phone,
meaning it operates on 900/1800/1900 GSM frequencies and if it is
unlocked.
I hope this information helped. I'd be glad to answer any more
questions you might have, since I was born in Europe and spend a long
time of my life there.
Greetings,
Andy

Get an UNLOCKED tri-band GSM phone in the US and buy a pre-paid SIM
when you get to Europe.

I can't provide any help on what SIM to get but we have used
www.oneworldgift.com to buy GSM phones and were pleased with them.
The phones were new in the box and delivered quickly.  They have
phones from ~$99 to nearly $1,000.

-D