I haven't tried AppleTV's Airplay, but I think it just mirrors your desktop correct? So theoretically you could just run Steam's Big Picture Mode off of a Mac and use AirPlay to stream it to your TV?
OnLive's protocol seems to be far more willing to sacrifice frames and quality to keep everything in sync; where AirPlay will (in my experience) hold up the stream until every last frame gets through, sync be damned. Also, AirPlay doesn't modify apps or input at all to compensate for the lag to the TV.