"In anticipation of an upcoming announcement from Valve," was all we were told.

Whatever's brewing at Valve--there's all sorts of Portal wackiness underway, and there have been indications that its digital distribution platform Steam might be making the leap to Mac--we're likely to hear more soon, as GDC 2010 kicks off next week.

Update: Two additional images have surfaced via MacRumors and MacNN, furthering speculation that Valve is bringing Steam and its games to Mac.

Update 2: Yet another image has arrived--this one via Rock, Paper, Shotgun--a facsimile of old Apple advertisements that all but confirms Steam is coming to Mac.

Update 3: Eurogamer brings us the fifth of the six images, portraying Left 4 Dead's Francis alongside a rainbow-colored L4D logo and the words "I hate different."

Update 4: And here's the sixth image-- Half-Life 2's Alyx recreating the classic 1984-inspired Macintosh television ad--courtesy of Macworld.