You may remember that Apple actually approached Verizon first about being the exclusive iPhone carrier. Verizon, realizing that having a incredibly successful phone would bring network traffic to a screeching halt, wisely said, "Nah, we're cool with the phones we have." So Apple cruised on over to Cingular and hammered out a deal with them. Then Cingular became AT&T, and we all bought iPhones and broke the network in major metropolitan areas.
Anywho, in the interest of posterity, maclife.com have compiled a handy infograph to outline the current status of the iPhone on Verizon below.