There was a recent rumour that Bungie has been working on a PC version of Halo 3. The rumour sprung when it was discovered that Halo 3 was mentioned in the code of a recent AMD graphics driver.
Microsoft has officially released a statement debunking the rumour.
“We currently do not have plans to release any Halo titles on Steam, nor do we currently have plans to release a PC version of Halo 3,” Microsoft told Eurogamer.
However, this doesn’t really address why Halo 3 had an entry in the AMD driver.