Originally Halo was supposed to be a badass third person shooter that was just going to kick complete and total ass. It was (from what I've heard) developed by programmers on P200s to ensure that it didn't fall into the "requires an 18Ghz Dual P5" syndrome. It was going to be released simultaniously for PC, Mac and Linux. Then Microsoft bought the company, or the game, depending on who you talk to, to make it the premier title for the X-Box. This of course meant that the team had to re-do a lot, changing and adding a lot for the X-Box architecture, and of course the nice zippy graphics card and processor the X-Box offered.
At that time I knew we'd never see a Linux or Mac version, and I was very very doubtful we'd see a PC version. Howver, there is a story or GameSpy from the company (Gearbox Software) that got the job of doing the PC port. Good to see there is still a chance.
On a personal note, I'm sad to say I don't think that Halo matters anymore. If it had come out in 1999 or whever it was it was first talked about it would have been the best game ever (or something like that), but this many years later, there are other games that do what it does, and it has been leapfrogged technologically, so I don't think the impact it could have made will still be there. My hopes are still out that it'll be good though.