If it is being developed it isn’t being done externally
343 Industries, which is the Microsoft internal Halo studio, has let it be known that if an HD version of Halo Combat Evolved is being developed it isn’t happening externally. The rumors that Saber Interactive was working on this project are apparently false.
343 has worked with an external studio previously (for example, on the Defiant map pack which was done by Certain Affinity, which is a studio primarily of ex-Bungie staffers) and it was a natural fit. No one would seem to confirm if a re-make of the original Combat Evolved HD is really happening.
Right now, we think there is something going on, but we don’t exactly know what it is. Our sources continue to think it is going to happen, but it would seem certain that 343 wants to turn down the expectations of this possibility for the time being.