Looks like that is the case
Rumors over the weekend have led a number of sources to tell us that Microsoft is planning to release Halo 3 on the PC. If accurate, we are also hearing in a surprising move that Microsoft is planning to release it using both their Games for Windows platform as well as offering it via Steam.
In the past, Microsoft has preferred to use their Game for Windows digital distribution platform to deliver their games rather than using Steam, but it could be that the decision was made to offer it via Steam, as well, because they want to maximize sales of Halo 3 on the PC. After all, it has been a long time since the game was previously released on the Xbox 360.
If Halo 3 is coming to the PC, the question is when will it happen? We would guess that it will be sooner than later; but with the release of a new console, we suspect that Microsoft will avoid the announcement of the new title near the announcement of the new console.