Never really cared about the Halo games when they came out, I might pick them up if they make it to Steam (and go on sale eventually, of course). I mean, might as well: They're a pretty big part of the way FPS's evolved. That said, seems unlikely that this is happening. I'm sure Microsoft would make Halo 3 going on PC a pretty big event, not something that accidentally appears on Steam Database. As pointed out above, Quantum Conundrum 2 doesn't seem too likely either, the first one didn't do that well (Unless it's been announced and I somehow managed to completely miss the news, which isn't beyond me).