dialing back

For no initial fee, Mobal will send you a global SIM card for your GSM mobile phone with a UK number. It gets attached to the credit card that you provide and you're billed as you use the service. The rates are pretty high (GPRS rates), but it claims to work everywhere and it doesn't expire.

If you're willing to shell out $60 US in startup cost (resulting in $20 worth of service credit), you can get the freakishly branded Global Riiing SIM, which has better rates (free incoming in most of Europe). If you don't add some money to the account every 9 months, it does expire.

I hate making decisions based on the convenient reasoning of amortization so I opted for the Mobal's "the first one is free" SIM.

The Traveler Insider has a reasonably detailed 7 part series on international cellphone options and they finally settle on the Global Riiing SIM.

Other options & research:
Tavel Insider's DIY decision matrix
NYT | Using Your Cellphone Anywhere in the World
Hop Mobile: the rates aren't good, but you get a number in Monaco!
In Touch Smart Cards: several identical options with inscrutable benefit
Cellular Abroad