via Aeropause.com

"One of the updates that was added in the last Games for Windows LIVE client update was the ability for developers to create an in-game marketplace front end. The first game that has seemed to take advantage of this is Batman: Arkham Asylum.

Thursday, we had the announced free DLC for Batman: Arkham Asylum released, so I decided to take the plunge and download the DLC via the in-game marketplace. While you can download it via the standalone client, I figured why leave the game if I can get it in there."