In addition to some older titles, it would seem the sequel to 2007's Portal will also be available for the Mac. Portal 2 was unveiled as the cover story for Game Informer magazine a few days ago, and at the time, only cover shots were available. More pages have now leaked, and one lists the platforms that Portal 2 will launch on: Xbox 360, PC, and Mac.
You can view all of the page scans here (scroll about halfway down for image links).
Update: Valve has issued an official statement and as it turns out, Steam and numerous Source games will launch on Mac in April, not May. Additionally, select titles purchased for Mac or PC will be playable on both platforms free of charge via a new feature called Steam Play. Portal 2 is also confirmed to be the first simultaneous release for Mac and PC.