http://uk.xbox360.ig.../1217705p1.html
apparently it is for 29th February, so excited.
EDIT: THIS IS ALL JUST A RUMOR, DON'T GET HOPES UP TOO HIGH.
"For now, nothing has been confirmed and this can only be taken as a rumor. Microsoft does typically hold a Spring Showcase in San Francisco ahead of the Game Developer's Conference, and this year's has already been announced, though Halo 4 hasn't been mentioned in connection with it."