My new plan, since AT&T no longer pro-rates international data is as follows.

My iPhone 4S (still under contract and locked) goes into Airplane mode with only WiFi on.

My inexpensive unlocked Android Galaxy Y gets a local SIM with voice and data plans activated in the local country. I use WiFi as much as I can, and data when I can't for calling and SMS.

I setup the WiFi hotspot in the Galaxy Y to allow the 4S to tether and use it's cellular data when away from work or hotel where there's usually WiFi.

Google Voice, Vonage Mobile, GrooveIP, Talkatone as well as other VOIP and SMS over data apps are the rest of this. I like being able to use either IOS or Android apps with this setup.

I carry my older now out of contract and unlocked by AT&T iPhone 3G as a backup in case there's an issue with one of the other phones.