From Apple’s site:
If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone 4 is the best choice. It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM carrier worldwide.
If I keeping doing as much international traveling as I have been, I would definitely consider buying one. One international cell phone bill alone could easily pay for the steep $650 price tag.
This is also yet another (slight) screwing of AT&T by Apple. Which I love.