Files found in the program's folders point to yes.

February 25, 2010 - Valve's Steam platform has become one of the biggest engines driving digital distribution, powering PC gaming and providing a model for online distribution in general. But Mac users have mostly been kept out of the picture, unless they've been persistent enough to install Boot Camp or a Windows emulator. Now it looks like Valve is finally bringing Steam to the Mac. 

Folks from the Steam forums (via Engadget) poked around the folders in the latest Steam beta release, and they came across a few interesting images. Each filename begins with "osx" and the images include dock and window icons. There's also an "osx.menu" file featuring coding for the Steam menu functions.

Of course, it's possible Valve is just working on getting the Steam platform running on Macs, without actually porting any of their current games. The change could be as small as simply offering current Mac games for download. Still, it's a big step and one that will hopefully lead to some Half-Life goodness on Macs across the world.

http://gear.ign.com/articles/107/1072295p1.html

On the Endgadget article it references:

"But, play portal with one mouse button or will the mac version only have one coloured portal?? :P"

This made me laugh so hard. :D

But, more on topic... this really won't change a thing since OSX doesn't support DX, and I don't really see any big push to switch to OpenGL. =\

I don't really see the point of an OSX client. The Mac market share is only around 5% world wide, they aren't really gaming machines as most of the consumer models (imacs and MacBooks) have very weak graphics cards, plus they can run Windows anyway.

Direct 2 Drive sells Mac games so I don't see why Steam can't. But I doubt Valve would port any of their games to Mac though.

Valve games run great on mac osx through cider/wine/crossover wrappers... if valve want to they can port their games..

But why would they want to when you can already play them on Macs anyway and the potential market is tiny?

Wait... Steam doesn't work on Mac?

Mac is the best operational system ever. I really don't miss a wider support since the only pc games i care(the sims and Blizzard games) are avalieble to the system.

'Mac' isn't an 'operational system'. You're thinking of OSX, which has its good and bad points, just like Linux and Windows.