Update: It looks like Halo isn't coming after all.
"We currently do not have plans to release any 'Halo' titles on Steam," said a Microsoft spokesperson in response to our query.
While most developers aren't willing to disclose what their game's status is regarding Steam, Dyad creator Shawn McGrath has confirmed to Joystiq that his psychedelic PSN shooter would be making the leap. "This was a rather interesting thing that happened," McGrath said. "I didn't plan on announcing Dyad on Steam yet, but what can I do now?"
Elsewhere, Fez creator Phil Fish simply responded to my query with a simple, "no comment for now!" response, but given that Fish has said he'd like to make Fez multiplatform, the Steam version seems very likely.