I think it's quite possible Valve investing in OpenGL could actually be a PC focused plan with Mac support being an additional benefit.
http://store.steampowered.com/hwsurvey/
Valve's games and the Source Engine are very scalable and looking at their user base on the Steam Hardware Survey reflects this. I don't doubt Valve will want to maintain as broad a user base as possible as they make improvements to the Source Engine. Most notably, the Source Engine doesn't have a DX10 codepath and use DX10 features yet. The most interesting statistic though is that even though 49% of Steam gamers have DX10 capable GPU with a DX10 capable OS (Vista or Windows 7), a further 27% of gamers have a DX10 capable GPU but are using Windows XP. In other words, if Valve were to write a DX10 code path, 1 third of users who have DX10 hardware would not take advantage of it. In contrast, if Valve used OpenGL 3.x to take full advantage of DX10 and newer GPUs, 100% of DX10 GPU owners regardless whether they are using Vista, Windows 7, XP or OS X can see the new effects in the Source Engine. Adding OpenGL support seems like the best way for Valve to efficiently target new features to more of their users.
Conveniently, OS X seems to finally be gaining OpenGL 3.0 support in 10.6.3. Valve would probably need to add an OpenGL 2.x codepath to support Leopard, which is still the most popular version of OS X, and older DX9 GPUs in Snow Leopard, and an OpenGL 3.x codepath for DX10 GPUs in Snow Leopard.
Half-Life 2, portal, TF2, etc. all share the same engine. If Portal Runs on Mac, the entire stable will.
As I said in my earlier post, that isn't necessarily true. Half Life 2 and others are available on the PS3, but the Source Engine itself can't output code to the PS3 unlike the PC and XBox 360 according to Valve's Source Engine marketing materials. The PS3 games were ported separately by EA and not by Valve. We have to hope the Source Engine has been ported natively to output code for OS X rather than them selecting a few games to port.