Steve of Forerunners.org has supplied some of the bits of information he's been able to glean while down at PC Halo Fanstock at Gearbox Software in Plano, TX.

• PC and Mac versions of Halo will not be released simultaneously. The Macintosh version is not yet as far along as the PC version. (Westlake Interactive still lists the Mac version as "pre-Alpha" as of this writing.)
• Level editors will not ship with the game, but be downloadable from Gearbox, along with (hopefully) some tutorials.
• It is unknown at this time whether there will be a Mac version of the editors.
• The Halo editor is comprised of three programs.
• The Halo level editor is called "Sapien".
• The shapes and physics editor is called "Guerilla".
• PC Halo has six new multiplayer maps. Many draw on the original game's solo maps for inspiration. They are:
  • Death Island
  • Timberland
  • Ice Fields
  • Danger Canyon
  • Gephyrophobia
  • Infinity
• PC Halo is one of the quickest loading PC games ever. It is reported to load in approximately three seconds.
• PC Halo has two weapons not in the original (unhacked) Xbox release: the flamethrower and the fuel rod gun.
• User-created netlevels will be about 18Mb in size.
• User-created solo levels can be up to about 100 Mb.
• System requirements under Windows will be around a 700 Mhz processor (assuming P3 or P4, not sure), a "good" video card, and 128Mb of free RAM.
• There is a new version of the Warthog with three cannons on top.
• After playing a bit of Halo on the PC, the guys got to watch the same realtime demo of Halo 2 given at E3, and got to play a little Halo 2 themselves on Xbox dev kits. Observers report that Halo 2 is graphically much improved over the original.
• PC Halo will ship on 1 CD, not on a DVD as the Xbox original did.

We will update this item throughout the day as more information becomes available.