Unless it is a world phone, the upside is limited
The biggest question is whether a Verizon iPhone will be a true "World Phone" like their Blackberry offerings that have both CDMA & GSM radios. I tend to doubt it just because of the limited real estate available in the iPhone 4 chassis. Adding a GSM radio would seem to require
a. antenna redesign (already has 3 for the GSM bands, how do you add CDMA to that?)
b. another chip inside (meaning removal of...something -- battery life, memory, etc)
If Verizon manages to pull off a truly world-capable iPhone, great! I had Verizon service for years until getting my iPhone 4, and theytruly have better service throughout North America. But, go to Europe or to most parts of Asia (Japan being a notable exception) and CDMA is worthless.
This is the same issue Verizon has with their Android devices, BTW -- no world capable phone. World travelers may be a relatively small device market, but I would guess they are relatively high-end when it comes to service plans.