Speculation

[Guardian]

So apparently a list of games appeared in a list thingy to do with steam. Halo, Halo 2, Halo 3 were on it... Tbh I haven't actually read the whole article, just the bit saying it was there and Microsoft denies it. So yeah read into it what you like.http://www.ausgamers.com/news/read/3271747/pc-release-speculation-spurred-as-dyad-halo-3-and-more-appear-in-steam-apps-database#Comments

[arbiter901]

Theres already a thread on this.
But yeah it makes no sense for valve to add these titles to the registry.