Officially unveiled and announced at the GDC awards just now, they've released a trailer. Nothing too special here, just confirming it's existence and what appears to be a release date of 10th November 2009. Brilliant news
(Man, naming this thread was a pain. They appear to have removed the Call of Duty bit from the title but it'll be known as a CoD game for sure. I imagine it'll be Call of Duty: Modern Warfare 2. In fact, can a mod remove the 4 from the title please? )
Trailer here: http://modernwarfare2.infinityward.com/