Spong says its sources at Microsoft indicate that a new, non-Halo Bungie game will be announced at X06 very soon:

Speaking to a Microsoft source close to the management of the entire Xbox 360 project today, we were told that, "As far as things stand, Halo 3 is the last game [in the series] and it's more than just 'a thing to say' which I know came as a surprise to everyone [at Microsoft Game Studios not involved with the decision-making process] when it was announced. A new game from Bungie, not Halo 4, will be announced soon and pre-production has begun. So if you're looking for what to expect at X06, you should be looking at that."

It seems Spong doesn't really believe Halo 3 is the last Halo game, though, as they suggest that George Lucas-style a whole new trilogy will be announced later.

X06 will be held on September 27 and 28 of this year in Barcelona, Spain.

As always, keep your sodium chloride handy while reading.