OK, so my wife and I are traveling through-out Europe in a few months. We would like to have a phone to be able to call hotels and such, we are spending about 15 days in UK and Ireland then about 15 days in between Paris, Germany, Austria, and Italy. We have verizon, but they are way spendy to take along. We were trying to find pre-paid phones that are versatile that work over there. It is more important for them to work in the UK and Ireland (we are driving in Ireland) than the rest, but it would be nice.
You need an unlocked tri-band or quad-band GSM phone. Unlocked meaning it's not forced to use only one carrier's SIM cards, which is the case if the phone is bought at Cingular or T-Mobile shops. You can buy these on Ebay. phonescoop.com will help you find what phone models are GSM 900/1800 (the frequencies in Europe).
Then, with an unlocked phone you can simply buy a pre-paid chip from a merchant in Europe. You'll pay roaming charges when you change countries. As an alternative, you could buy a pre-paid account from telestial.com or other site. Just search for global prepaid SIM. The sites will also sell cheap unlocked phones to use.
A friend of mine just rented an international cell phone for her daughter who is in Italy for three months. You could google- "international cell phones" AND renting - or I'll try to get ahole of her and get the details to you tomorrow. I know she was short on time and drove to Seattle to get it. They have been very pleased with it.