No it wasn't.

Could it be slimline 360?

Highly unlikely I think, they wouldn't have introduced the 60GB model if they were going to reveal a Slimline one so soon.

It wasn't announced at the convention. The new Halo will be a squad based shooter, which delves more into the lives of the guys below Master Chief. It was announced by Microsoft representative Mattrick at E3 though. They kept it simple and also Bungie wasn't at E3. Microsoft had to appologize for not giving it enough press, but at the same time it was probably because there weren't any screens or videos to show.

Proof: http://www.gameguru.in/microsoft-xbox-360/2008/17/microsoft-speaks-about-new-halo-games-no-show-at-e3-2008/

The squad based information was based entirely on a rumor, it has neither been confirmed or denied.  And techincally it was never announced.  Bungie was going to announce their new project(timer on their website indicating when), but M$ told them to pull back what they had.  M$ stated that they wanted to announce it at a time of their choosing.  No official information has been announced about Bungie's new project.

You are in denial. These things were confirmed. Don Mattrick announced them so they ARE NOT rumors, just made by some fanboy.

Quotations:

"The saga of Bungie v. Microsoft in The Case of the Missing Haloes continues. The L.A. Times spoke with Xbox senior VP Don Mattrick regarding the omission of Bungie's announcement from its Monday press conference. Mattrick told the writers that Microsoft decided to nix the reveal during the show to "save time."

Let that sink in for a moment, you 5+ million Halo fans. Time. Word around the campfire is Microsoft's conference was originally set to run for two-and-a-half hours, but Mattrick wanted to cut the show down to 90 minutes. Of the items to be cut from the show? Anything named Halo. Mattrick told the paper that Microsoft's conference already contained, "an embarrassment of riches," so the Halo trump cards were removed from the docket."

This is the reason why it wasn't up on stage at the convention. He said it during an interview later on.

A little more for you to read:http://www.xbox360fanboy.com/tag/don-mattrick/

The squad based information was based entirely on a rumor, it has neither been confirmed or denied. And techincally it was never announced. Bungie was going to announce their new project(timer on their website indicating when), but M$ told them to pull back what they had. M$ stated that they wanted to announce it at a time of their choosing. No official information has been announced about Bungie's new project.

Once Bungie.net comes back online, I will give you their side of the story.

Give a link to the squad based information.

 Yes...they decided not to show Halo as to not overshadow Gears 2.

My contention with what you said is the fact that you claim the new Halo was announced, and thus no need to announce it at PAX. It was not officially announced, there is no official information regarding the game(squad based info is mere speculation), and the links and quote you provided merely showed the fact that there is a new Halo game in the works(which is quite obvious). Bungie has stated they have no plans for announcing their new project at PAX.

Yeah I guess the source was credible they say, but it wasn't confirmed by MS. This is all we have to hang onto for now.

"What we heard: Last year, those skilled enough to complete Halo 3 on legendary difficulty were treated to an extended ending of the game after the credits rolled. For those who don't have a knack for Covenant slaying, numerous YouTube videos depict how, after the defeat of the Flood, series hero the Master Chief is left adrift in space in half of the United Nations Space Command (UNSC) ship Forward Unto Dawn.

After a brief conversation with the AI Cortana, the genetically enhanced, perpetually armored Chief enters a hypersleep chamber. The cutscene camera switches to an exterior shot, which pulls back to reveal the remnants of the Dawn entering orbit around a mystery planet. As the sun rises on the planet, music swells, and the screen fades to white, at which point the cutscene abruptly ends.

With that setup, many assumed that Bungie's next project would feature the Master Chief marooned on a strange planet, struggling for survival--a sort of off-world Far Cry. However, a source with knowledge of the studio's development slate told GameSpot a different story. She or he said that sometime this year, Bungie will announce a "darker, grittier" Halo game that doesn't feature the Master Chief at all.

According to the source, the new Halo game will be a spin-off that resembles "a cross between Ghost Recon and Gears of War." It will reportedly tell a "less cartoony" and "more bloody, violent, and grim" tale of a battle between UNSC forces and the pan-racial religious empire known as the Covenant. The game's atmosphere was described as similar to that of the live-action Halo shorts directed by South African filmmaker Neil Blomkamp (pictured), who was to helm the now-stalled big-screen adaptation of the game.

The source says that the upcoming Halo game is a tactical shooter that will follow a squad of colonial marines or orbital-drop shock troopers engaged in "intense" fighting on the ground. Gameplay details were scant, other than the fact that it will reportedly let gamers play as an entire squad in co-op mode through the entire campaign. The exact number of co-op players has yet to be finalized, but it could be as high as eight. It is unclear if the new game is the same as the "Halo: Forerunner" project first rumored in 2006.

The official story: Microsoft had not yet responded to requests for comment as of press time.

Bogus or not bogus?: Based on the strength of the source, this is almost certainly not bogus. And with Bungie's teaser last week, news about the game could arrive as soon as tomorrow's Microsoft press conference at the E3 Media & Business Summit. "

Site:http://www.gamespot.com/forums/show_blog_entry.php?topic_id=26483669