I second the recommendation for Skype. WiFi is relatively easy to find, and the costs are very low.

Also, if you plan on using your cell phone, call your providers Customer service ahead of time, for three reasons:

1. Sometimes the rates are much, much higher than you would expect for international roaming.

2. Many carriers don't enable international roaming unless you ask them to.

3. If your carrier doesn't use GSM ( only AT&T and T-Mobile in the U.S use this ), your options may be limited. Sprint and Verizon phones don't work in many non U.S. areas, unless you have one of their special dual-band phones.

Personally, I've always just gotten by with using my cell phone in a very limited way, and making longer calls using Skype. I've also had some luck using a PC softphone to connect to my home VOIP service. Not something I'd recommend to non-geeks though...