Boy Genius Report did a little digging around to find some concrete numbers for Verizon iPhone sales, and while these probably aren't wholly representative of the larger picture, they did release numbers purporting to be from five different Apple stores of the sales of iPhones on both AT&T and Verizon. You can see the day-to-day-breakdown on BGR, but the totals are a little surprising. Over the five-day period, Apple sold 3,009 iPhones with AT&T service, and 3,992 with Verizon service. Obviously, the Verizon iPhones are selling better, but when you consider that this is the first few days of the phones on sale (and the expectations going into the new service), it's interesting that the numbers aren't higher.
Now for the caveats: with an extended pre-order period for existing Verizon customers that apparently did pretty well, and the option of online ordering a day prior to in-person sales, there's not much of a hook here. Apple and Verizon could have sold (and probably did) hundreds of thousands of phones before the opening moments. Also, the iPhone 4 is a known quantity on the market at this point, and adding a carrier (which has happened in scores of countries before) is not necessarily as big of a splash as a new introduction. Yes, there has been pent-up demand for the Verizon iPhone, but a lot of people might have given up waiting in the interim -- or are still waiting for their contract re-up date to roll around instead of paying more than $600 for the unsubsidized phones.
We'll wait until we see official numbers to make any pronouncements, but BGR's source says the sales are below Apple's expectations. The source also says that only 14 percent of those buying the Verizon iPhone were AT&T users. Before the phone went on sale, much of the thinking about a "vPhone" was that it would steal customers from AT&T, but that may not be the case. Two things may be happening here: AT&T likely still has an audience and will continue to sell iPhones, and new Verizon iPhone customers may be coming from other smartphone or feature phone platforms completely.