I'm traveling to Europe tomorrow, and was checking out sim card options for my Samsung Vibrant...
Truphone has just reduced their rates, by taking away the monthly $15 "membership" fee, so the low rates are now standard in the US, UK, and Australia. see
http://www.truphone.com/en-AU/Produc.../Rate-Checker/
You can also buy a local number in these 3 countries for $8 a month (and cancel as soon as you don't need it).
of course, it's still more than skype, but competitive with locally available sim cards - I've usually used Orange for UK trips.