It is assumed that the device will be on AT&T, because AT&T has been the biggest supporter of Windows Phone devices among US carriers, and AT&T needs more flagship devices to push its newly forming LTE network. However, it is also possible that there will be a Verizon variant of the Lumia 900, because Verzion has said that it wanted to see more LTE devices from Windows Phone in order to support the platform. This may be just the reason the carrier needs. We'll have to wait until CES to find out though, because that is widely expected to be when the device is formally announced.
The Nokia Lumia 900 is expected to have a 4.3" Clear Black AMOLED display, 16 or 32 GB models, an 8 MP rear camera, a front camera, and the same unibody design of the Lumia 800. The new information also puts the device dimensions at 125.5 x 65.8 x 9mm (4.9 x 2.6 x .35"), which would make it much larger, but also much thinner than the Lumia 800, which is over 12mm thick.
source: Nokia Gadgets