There are a handful of Samsung phones that support both CDMA (when you're in the U.S.) and GSM (when elsewhere). In theory, you could get one from Verizon or Sprint.

In practice, the roaming rates they'll charge you are insane. Unless you're a business traveller who must have the same number, everywhere you go, it's probably much more effective to borrow somebody else's GSM phone when you travel internationally and get some kind of local pre-paid card to use with it. Of course, pre-paid card prices and such vary radically from one country to the next.