Revealed at Microsoft's 2009 E3 press conference, the first two downloadable map packs for Inifinity Ward's Modern Warfare 2 will be timed Xbox 360 exclusives.
The news was delivered by Microsoft's John Schappert following a gameplay demonstration of the game involving climbing up the side of an icy mountain, which is perhaps an appropriate parallel to today's gamers trying to keep up with all of these timed console exclusives.
No details were given regarding the content of the map packs, but I suppose we can wait until we have the actual game before we start worrying about DLC.