Being that Valve has their own time, late April really meant today.

Seems to work just like Steam on Windows. I've added non Steam games to the list, and the overlay works in them as well. Looks to be 60ish titles on the store already, though the only Valve made game is Portal.

Also, Portal is free until May 24th, for Mac or Windows users.
http://store.steampowered.com/freeportal/