No titles announced yet, but there will be some soon, and also I guess there's the byproduct of Source coming to 360:
http://www.next-gen.biz/index.php?option=com_content&task=view&id=2831&Itemid=2
http://www.next-gen.biz/index.php?option=com_content&task=view&id=2831&Itemid=2
The vague announcement proclaimed that Valve's first Xbox 360 title is "coming soon", and more than one title is planned for the console. The company assured that all of the games will use Valve's impressive Source engine, which was used in Half-Life 2.
Valve president and co-founder Gabe Newell said, "The combination of Source and the 360 provides game designers the chance to create powerful entertainment experiences. Whether developing a traditional FPS, RTS, RPG or delving into new genres, the Xbox 360 is a great platform for expanding Source and our game experiences."