Microsoft sure doesn't skimp when it comes to marketing Halo... The ODST commercial is badass.
Microsoft was going to do a series of movies with Peter Jackson (King Kong, LOTR movies) but they had conflicting ideas so they took the project off the table. Now it's definitely not happening anytime soon (at least with Peter Jackson) because he's doing the Distric line of movies. And by the time Microsoft does get around to aprroving a Halo movie, the game franchise will be over and no one will actually care about a movie.
It's a shame too because if it turned out to be anything like the ODST commercial, it would've grossed 3907309473 billion dollars by gamers watching it 4 times in the theater.
Halo series of games has a good plot line which would make for a great trio of movies. And with Microsofts deep pockets, the movie(s) certainly wouldn't suck.