@Frogboy
I don't quite understand the logic of "not thinking to do your gaming on Mac" ...
Hardware wise, Mac is virtually identical to PC these days while input devices (mouse and keyboard) are same as well...
As a gamer, I cant care less on what my game is running as long as I am having smooth experience and am not forced to use input devices I don't like (read console controllers)...
Only difference between Mac and PC is OS and most (if not all) developers would find developing for Mac way easier due to Apple's "closed eco system" - meaning less bugs, less time waste optimising your game for trillion possible PC configurations, more stable final product and ultimately smoother end-user experience...
Anyway...
Regarding Valve - my feeling is that there is much more behind all this than what's been reported so far... Yes, Steam is coming to Mac and yes Mac users will soon be able to play HL, TF, L4D, Portal and other games... But my 6th sense is telling me that Apple together with other major publishers like EA, Valve, etc... is planing something for the long term future as well... Perhaps some sort of Apple console? Perhaps full gaming compatibility between desktop/laptop - Apple TV / Console - and future revisions of upcoming iPad?
Imagine starting game of Elemental or Sins on your desktop during lunch hour break, saving your progress and continuing the same game from your living room when you get back home or indeed in the train using iPad...
Crazy ideas that will most likely never happen - but I am still positive that there is much more behind this Apple / Valve deal than what we are allowed to see at the moment 
As for why Mac instead of PC - my honest answer is that I am simply tired... Tired of spending half of my time patching or updating either windows, or drivers or anti virus software or indeed buggy game releases...
Saying that, Mac isn't without its own problems - not at all!, but compared to Windows its really breath of fresh air when it comes down to user experience.
All the best with hopes that one day we see Impulse and Sins running on Mac