Call of Duty 4: Modern Warfare 2

"At this time, we are aware of the issues regarding the Title update for MW2 and the Map Pack.

Right now, the Map Pack is available for purchase but the title update that the game requires is still processing across Xbox LIVE worldwide. If you receive a prompt to receive the update, but the update fails you will need to try again later to receive the update. We are working hard to get this fixed and hope to have everything back to normal soon."

From Xbox team member pearl jam on the xbox forums.