Yes and no.
Originally Posted by MiaMike
Yes, in that it's entirely possible there will be one single iPhone that has both GSM and CDMA radios/chips. Meaning it will be a truly global phone.
No, if you buy an iPhone 5 from Verizon that has a GSM radio you can bet that it will be locked to Verizon. You would need to get a SIM from Verizon to use with an international roaming data plan.
Now, that said we don't know about jailbreaking and unlocking through unofficial means at this time. So maybe you will be able to at some point. There also could still be different models that work on different GSM bands. The Verizon iPhone 5 might not support AT&T's GSM bands and only support bands outside the USA.
In summary, we just don't know.