I've heard the Droid doesn't work internationally. Is that true of voice and data or just voice?
Droid doesn't work internationally because it's a CDMA phone and many parts of the world are very GSM dominant. If you're in a part of the world with CDMA coverage then you might be fine. If you're in a GSM dominant area, you'll be out of luck.
Yes, it will work in Cancun.
I seem to remember there being some benefit to calling Verizon and telling them when you plan to travel to get on an international roaming plan of sorts. Not required but makes calls cheaper. International roaming is charged per minute and not included in your monthly minute allowance.
I could be wrong on that but I'm pretty sure I read that in one of the few fine prints I actually read on a contract. I am certain that it will work in Cancun however.
Agree if you want to use a phone out of the USA and Parts of Canada. you need a gsm phone. troll the net you can rent them with pre-paid time and everything.