Not Likely as Verizon CDMA phones that are capable of GSM world phones roam on GSM with a seperate chip and sim card in the phone. I think Sprint will lock you in, just as AT&T and VZ did in the beginning. I do however see the next iPhone as being on phone that is on all platforms and sold Unlocked, but I do not make those decisions.
Besides you can buy an Unlocked for use in the USA, but you will pay full retail and for my iPhone 4 that would be in the area of $749.00 so not worth the cost especially if you lose the phone abroad, Yikes, Gulp your out Big Bucks.