It should be making its appearance in the US market most probably possibly under At&t.
Lumia 800 is already on sale in most parts of the UK and europe. and its a HUGE hit according to carriers over there.
Though you can get this phone at amazon, expansys or ebay, but at a pricey $699. it should have a heavily subsidized price once it hits at&t shelves in mar/apr.
This is one of the devices Nokia's going to announce at the CES2012 alongside other rumoured devices like the Nokia Ace.
sources: engadget; FCC