Microsoft staff have been hinting in multiple places that there will be a pretty big announcement for Halo 4 at some point today, which could potentially be the game's release date.
343 Industries, who're developing in the game, recently did a massive splurge of information in Game Informer, where they announced a new co-operative mode called Spartan Ops, which would be replacing Firefight.
One of the senior producers at 343 Industries, Alison Stroll, tweeted "A little birdy (or was it Andy?) told us @TeamCoco will have some big Halo 4 news tomorrow. Stay tuned! incoming wakeupjon."
This was also followed by a tweet from Microsoft's own Major Nelson, who said "Look for Halo4 news on the way in the next 24 hours. Hint: @TeamCoco plays a part!"
Sources in America are claiming that the release date will be the 6th of November, 2012 and that the news will be announced before 5pm today GMT.