We don't know that. Among the many rumors is that Apple is getting a combined CDMA/GSM chip from Qualcomm. If it supports GSM, then you can use it overseas, just like the other dual-standard phones Verizon and Sprint offer.
I don't know if Verizon supplies SIM cards for those, or if you're on your own to find a SIM and service overseas.
Verizon (and sister company Vodaphone) provides the SIM for the world-edition Blackberries (Tour, Bold, Storm) so I doubt this would be any different.
Of course, you can still use a local SIM to avoid roaming.