RUMOR ALERT:
http://www.joystiq.com/2011/02/04/halo-combat-evolved-hd/
Halo 1 HD with online co-op targeted for this fall.
I don't know if I trust Joystiq's source on this one, but it's worth discussing.
The real surprise here is that 343 is NOT the one developing it - they're basically acting as a publisher, it seems.
The dudes behind Timeshift are allegedly doing it and not using the Reach engine (I thought they were the Section 8 guys?)
Also, the one part of this that does seem believable is not having multiplayer. I doubt MS would go ahead with remaking it with full MP, since it would cannibalize what little pull Reach still has.