We've known that The Team Formerly Known as Myth II (TTFKAM2) has been working on something for awhile. Something pointed out by HWED at the Myth Townhall is that UGR has some interesting items in its release date listing:

Now we [should] all know how much we can trust release dates... especially those that don't publicly come from the developer themselves, but it is interesting to see Myth III included in their listing. Do they know something we don't? Well, not being satisfied with this curious little blurb of information, HWED sent off an inquiring E-mail to the Editor of UGR and received the following in his reply:

...Bungie plans to release Oni and Halo later this year and an inside source told us that Myth III is tentatively planned as a late 2001 title...

In related news, GameSpot is currently running an article titled, Which Game Do you Think Most Deserves a Sequel? and includes Myth III in its listing. Also, Bungie.Org has an interesting link posted at the top of their main page. It currently says ???.bungie.org and doesn't go anywhere.