Here's a link:

http://multiplayerblog.mtv.com/2008/07/16/microsoft-confirms-bungie-is-developing-new-halo

It's not the "Superintendent" project that wasn't announced today as planned.  Something else.

Yay.  Halo is good.

why would Microsoft pull it down?

Because it is a false claim. Making false claims can result in companies being sued.

So we have three Halo games in development. Yummy!

No. It's most likely because of:

a) Microsoft killed everyone with the XIII announcement. No need to plunge the dagger further
b) The project is very similar to MAG, and Microsoft pulled it due to something.

It could be both, but I'm going with #2. Microsoft has been working on Project DonnyBrook for about a year now, and I am sure that they want to reveal, and counter something with this.

So either the game pales in comparison to MAG, and they wanted to save face, or the game is better than MAG, but wanted to reveal it later.

I also think that Microsoft wanted to show the game world that "Halo" doesn't need to be its main focus and once again it can survive without going to their "Ace in the hole" Halo. They didn't even mention Halo Wars in the press conference.

Once again I give them major credit for being able to do that.

I am guessing B as well. 

If the big announcement was "Look, we have 256 player multiplayer!  More than any other game!", it would look be ignored because that was already announced by another game.

I really don't think the project is in competition with MAG. I've been a Bungie follower forever - pre-Halo - and I really didn't think they were going with a shooter as their first non-Halo project. Really.

Besides, the MAG trailer was cool, certainly, but it wasn't like that they showed ANY reason to believe that game was anywhere near completion or even playable in any form. It was just a CG trailer showing the concept. The guy in the show even stressed that it was a REALLY early look at it.

I can't imagine they'd pull it because it couldn't compete with something that hasn't even made an appearance yet.

Plus, Bungie has always felt that anything beyond 6 v 6 is too large - that players don't have enough of an effect on the outcome to keep things interesting for the player. They have said this repeatedly and forever. It's why Halo never grew betond Big Team 8v8. I can't imagine the company would change its whole philosophy overnight and suddenly want 256-player battles.

I think that MS wants to keep some of there "supprises" for later on spread them out a little instead of letting everything out of the bag in one go(this might help sales). There also was a very good point being made maybe ms wanted to show that they are more than just halo and can cause a buzz without Halo being mentioned.