I recently traveled to Europe for a couple weeks R&R. I was concerned about the cost of making overseas phone calls, high data transfer fees, etc. And then at the very last minute I decided against bringing my MBP, which meant that I was totally dependent on the iPhone for my digital communication needs...
To make a long story short, it went pretty-much without a hitch at a total cost of less than $3.
When I boarded the non-stop flight from Portland to Amsterdam I disabled International Data Roaming, switched the iPhone to "Airplane" mode and enabled Wi-Fi. For the next 2 1/2 weeks I relied on Skype and private/public Wi-Fi. (Both of my apartment rentals had wireless Wi-Fi.) I put $10 on my Skype account and spent less than $3 calling friends in Europe and the U.S. and checking our voice mail once a day. I didn't incur any data roaming charges.
The only problem was with POP email using foreign ISPs. But at least one of my accounts did work most of the time (weird...) and there was always Web email options.
I would only add that I consider a public VPN account to be a must-have when traveling with a portable Mac and/or iPhone. I have used Witopia's fine service for years and their iPhone app worked flawlessly.