Valve has hinted at its upcoming announcement of Mac compatibility for Steam.
Pictures of Half-Life hero Gordon Freeman have surfaced with his Hazmat suit decked out in Apple colours and his crow bar blacked out, akin to the early iPod adverts.
Team Fortress 2 and Portal have also been pulled into the act with the respective turrets from each game doing a parody of the "I'm a mac... and I'm a PC" adverts (the clunky Team Fortress 2 turret is the PC). Even hard man Francis from Left 4 Dead appears in a parody advert.
So either this is the obvious link to a Mac compatibility announcement or warning that GLaDOS is actually an Apple product gone haywire.