Hasn't been confirmed yet but Reach DLC might be appearing soon since someone leaked a screen shot of the achievements screen for Reach and it showed 1750 points and 59 achievements.
Bungie says the image came during their optimization of the game and when the game is installed to the hard-drive but their website also makes no mention of any DLC for the future.
http://kotaku.com/5647933/halo-reach-dlc-tipped