Its official and it is coming out this summer.  Some highlights:

• Improved graphics
• 27 single-player stages from the Marathon universe
• 13 multiplayer levels
• Special survival game mode
• Split-screen mode for our players 
• 8-player gaming over Xbox live. Microsoft promises a new network codebase that will reduce lag in online games.

Read more here: http://xbox360.ign.com/articles/803/803291p1.html