Considering both Verizon and AT&T's LTE 4G implementations will likely be compatible, I'm thinking Apple will forgo dabbling into any CDMA compatible iPhone, regardless of its AT&T contracts.
http://www.engadget.com/2007/09/21/verizon-dumps-cdma-for-gsm-based-lte-in-4g-networks/
Moreover, I'd heard that the AT&T exclusivity wasn't as long an arrangement as Apple's "no CDMA" in the U.S. pledge. This would allow them to hook up with T-Mobile sooner than Verizon or Sprint. As soon as Verizon starts its roll-out, we'll start getting an idea of when it will be useable coverage, AT&T won't be too far behind, but its a way for the tables to turn, I think.
Verizon Wireless to begin LTE rollout in Q4 2009?
http://www.boygeniusreport.com/2008/12/10/verizon-to-begin-lte-rollout-in-q4-2009/
http://www.electronista.com/articles/09/02/17/att.on.4g.rollout/
~ CB
Did I say anywhere that I thought the iPhone would be coming to Verizon?