No, it looks like Halo 2.5 will be out in February. All it is is an ending and only available on Xbox Live.
Even after three years of work, Bungie was still unable to complete Halo 2. MS wouldn't grant them anymore time, and wanted it out on Novermber 9th. Bungie gave it a real great cliff hanger and sent it on it's way. (A friend at school told me this. It does make sence.)
Now about Halo 3. Halo 3 will be out with the Xbox 2 or shortly after. Thats what Bungie said. Did you last months issue of Game Informer? They said "Bungie is currently working on Halo 3 for Xbox 2" or something like that. I don't remember it exaclty.