IIRC, doesn't ATT's iPhone exclusivity end this summer? That's prime time to introduce a Verizon iPhone. I also believe Qualcomm has been working on a dual GSM/CDMA chip, which would make a single iPhone capable of being on ATT or Verizon much easier.
Also, ATT and Verizon are both switching to the LTE "4G" technology in the future, which completely removes the need to make two different phones. It's literally not if, but when, Verizon gets the iPhone. I'm not saying it's going to be this year, but Apple is a business and it's a very good business decision in the long run to get the iPhone to as many people as possible. And with multiple carriers switching to the same wireless technology (and so many people complaining about ATT these days), it's just a bad decision to keep with one carrier. Verizon wanted things in the past that Apple wasn't willing to give them. But Verizon is the one that's going to benefit more from having the iPhone, so I think they'll cave and give Apple what they want.
Some think the contract was renewed w/ the iPad agreement. But nobody knows for sure.
I'm hoping that's not the case.