The achievement set for Halo: Reach lists 49 achievements for a total Gamerscore of 1000. That there screen, in the latest Bungie Weekly Update, adds 10 cheevs and 250 Gamerscore. DLC inbound?
This image came from an item on installing Halo: Reach to the Xbox 360's hard drive (Bungie says the game has been optimised for this.) There's no official (or other) mention of DLC on the site.
Bungie Weekly Update: 09/24/10 [Bungie, thanks Gabriel Z.]