We've learned from people familiar with the matter that Nokia's delayed entry into the US is partly due to the fact that it wants to attack with LTE — and that's a feature that Windows Phone 7.5 doesn't presently support. Our understanding is that Nokia might either launch with Tango — the code name for Windows Phone's next release — or with specific Mango-based code drops that include LTE functionality, but either way, AT&T figures prominently into the equation with a variant of the Lumia 800 that includes LTE on board. (Of course, if the rumors that the Lumia 710 might launch on T-Mobile USA are accurate, that particular device wouldn't be LTE-capable.)
With Nokia taking the time to integrate LTE for US consumption, it stands to reason that they're talking to Verizon as well, but we haven't heard anything new on that front. We'll update you when (and if) we do.