Halo 3 looks set for a PC release, and Quantum Conundrum 2 is a thing that's happening, if we can trust Steam's database.
It was among a number of listed titles which haven't been announced for Steam release, including La-Mulana, Duke Nukem 3D: Atomic Edition, Fez, Dyad, Cut the Rope, Shadow Warrior Complete, Halo 2, Halo: Combat Evolved, Angry Birds Space, Angry Birds Seasons and Second Life. Other titles listed include known upcoming PC ports like Resident Evil: Revelations.
According to early comments, Steam Community pages were available for many of these titles - a function xPaw's search tool has only recently introduced. The links no longer seem to function; either this was a fib or Valve cracked down on the leak quickly.
Although Microsoft has historically relied on Games for Windows Live rather than utilising Steam's more popular PC distribution system, Gamechup reports a Microsoft staffer's job description included a reference to the monetisation of Steam.
Steam registry leaks are generally proved correct in the long run, but sometimes update months or even years ahead of announce or release, as with Final Fantasy VII. Brütal Legend was spotted in there over the weekend.