It is widely speculated that a Verizon iPhone will be a very good thing for both the carrier and for Apple. Verizon's customer base is large and an iPhone launch is likely to generate quite a few sales for both companies as soon as the iPhone hits the shelves. Verizon has a bit of clout even where Apple is concerned, and while this is simply an educated guess on my part I wouldn't be surprised if the announcement next week includes a white iPhone.
The white iPhone 4 has long been a point of contention with customers, namely the lack of the option. Several reasons for Apple's lack of producing a much-desired white model of the phone have been given by Apple since the iPhone 4's launch, including the unlikely excuse that a white version is too hard to make. My take on the whole situation is that a more likely reason there is no white iPhone in AT&T's inventory is because Verizon has an exclusive with Apple for one.
Verizon is late to the iPhone party and while getting a CDMA version to sell will be a good thing, the carrier surely wants some sort of exclusive to hold over AT&T. What better exclusive than a white iPhone 4 that is available no where else?