As much as I hate to say it, I dont think it will come out for PC, and here is my reasonig behind this:
I know from first hand that the PC gaming scene is dying, slowly, but surely. All of the hardcore PC gamers that like to compete on the PC circuit and just in general play PC games are all getting old, including me. All of the old school awesome players that you used to hear about dont play anymore, and there are not alot of new talent joining the scene.
Alot of the kids these days are not interested in PC games, its all about the xbox, and like someone mentioned, they think that the 360 is their God and that it has alot better graphics than a PC, and in reality that is partly true and partly not.
You can build a great PC that will run Halo 3, Crysis, Gears of War, and so on with about 800 dollars or less. It not longer takes 5,000 dollars to build a beast, but why build one when you can buy one that will run any game released... for less money.
I was very good at Halo PC and very competitive, and I would give anything to see Reach be as popular as the first halo was on the pc side, but when kids start playing when they are like 7 or 8, have no interest in computers and would have no idea on how to build one, the 360 is the only machine they know.
I would like to see Reach for PC but at an economical view, it would not be a top seller, because of the slow fall of the PC scene. I also blame the fall of PC gaming on WoW, and the only reason I dont pray every night that WoW would shut down, is because there would be mass suicides.
NOTE: Keep in mind that this is my opinion, and everyone has the own, and I respect anyones view. :)