You'd think that, but I've notice precisely 0 more non-Valve games being released for Mac since Steam came to OS X. The only releases seem to be from companies who already did Mac ports, and indies (who are generally multi-platform).
And Steam has been pretty good at picking up pre-existing Mac ports, notably Civilization IV, Assassin's Creed 2, and several Star Wars games. I think Steam is also largely responsible for the Mac port of Civilization V being a first-class platform instead of the port being outsourced and late (though cross-platform multiplayer support took a while, and so did some of the DLC).