In an interview with the Official Xbox Magazine, Valve's Doug Lombardi mentioned that they would "certainly consider" releasing some of their catalogue on Xbox Live Arcade.
This could include games from Valve's stable including the original Half-Life games or first-party HL mods like Counter-Strike, Team Fortress Classic, or Day of Defeat.
Would you play Counter-Strike on Xbox Live Arcade? Did you play the Xbox version? Neither did we.
This is far from a confirmation, but Valve is already entering the XBLA market with Portal: Still Alive.