This information comes from a source at a rival wireless carrier and not from the Microsoft marketing documentation I've referenced repeatedly. I have not confirmed this with other sources.
According to this source, AT&T has a 45-day exclusive hold on the Lumia 900. After that time, other carriers can negotiate with Nokia for a version of the handset that will work on their networks. The source speculates that T-Mobile (and probably Verizon, possibly Sprint) will be getting some version of the 900 after it's been released on AT&T, most likely in the third quarter of 2012.These improved versions of the 900 will allegedly include an improved, 12 megapixel camera, the source revealed.
Via WinSupersite