According to the gaming blog, the remake of Halo will use all-new art assets, and will be based on a different engine than Halo: Reach. It will support 1080p and--reportedly--3D HDTVs, as well as online co-op. The original Halo: Combat Evolved only supported local split-screen co-op.
Joystiq says that the game will ship on November 15, 2011--10 years to the day after the original Halo released. The game is supposed to be one of two Halo titles in development, with the other in the works at 343 Industries, Microsoft's internal Halo-only studio. 343 has been on a hiring tear of late, posting listings for a variety of positions to work on the next big Halo game.
[Gamespot]