Regardless of when this happens, and it is "when" and not "if," iPhone volumes will surge upwards again as literally millions that have been sitting on the sidelines waiting for someone besides AT&T to carry the phone.
Verizon doesn't guarantee success of course. Many thought the Palm Pre would enjoy a resurgence when they added the Verizon network to their existing Sprint distribution channel, but the WebOS devices pretty much fizzled on the nations number one carrier.
The iPhone is a very different beast though. Every release brings renewed vigor to the platform. Version 4 sold over 1.5 million devices in a single day.
The day this happens though there will be a flood of iPhones hitting eBay and Craig's List as current AT&T subscribers put their devices up for sale and move over to Verizon, early termination fees be damned.
What you have to wonder though is will Verizon's network be any better? They have had three years to watch AT&T struggle under the demands of the iPhone's bandwidth appetite and have taken a number of shots at AT&T, like the most recent "There's a Map for That" campaign. To handle it, it means Verizon must be significantly overbuilt for its current needs. That is one reason it has such good reputation. Can they keep that reputation the day after they launch the iPhone?