Riot tells us that it's still coming. The game did get delayed longer than expected, but in an official statement, we're told that they "are putting some final touches on the Mac product and expect to ship by this fall." Season One was the priority, even while we were being shown the preview for the Mac version, but "now that it's out," Riot says, "we can focus on finalizing the Mac client and other upcoming features."
That sounds good. Since I first played the game for that preview, I've become a big fan; it's a quality version of the Defense of the Ancients mod for Warcraft III, and the fact that it's free to play on the PC (or in Bootcamp, if you can't wait any longer) is just icing on the cake. We'll keep an eye out for the Mac release and let you know when it's ready.