A gamer has sent in an image to VE3D that shows the 'Halo 3' logo part of a "Free trials at Microsoft Game Studios" advertisement.
The other two game titles shown are Zoo Tycoon and Gears of War - both available on PC. It would certainly be a boon for PC gamers and Halo fans to get their modding mitts on it.
An Xbox360 controller is in the ad too so it's likely referring to MS games in general, sadly. Gears of War 2 is set for release this year on Xbox360 exclusively so to announce Halo 3 for PC wouldn't hurt any.
Don't hold your breath.