Kotaku reports,

"Valve sent word five minutes ago that the public release of Steam for the Mac will be on May 12. No other details were included in the statement.

Originally due for release sometime in April, there was, of course, less than two days left to make that deadline. Valve gave no reason for the delay.

Steam for the Mac will bring with it Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal and the Half-Life series."

Source: http://kotaku.com/5527672/valve-dates-steam-for-mac-its-may-12