OK, I realize I sounded very much like a troll in my previous post, but none of the opposing arguments so far have convinced me otherwise. Rewriting the Steam client for OS X isn't a trivial task (although I'm sure it's nothing like trying to port a game), and they'd be doing it for a handful of existing casual games, essentially providing a place to keep them in a list (which I already do with a stack on my Dock) and keep track of how many hours you've spent on each (which, admittedly, stacks don't do).
I've played Ciderized games, and they're such pale, quivering shadows of their former Windows selves that I'm tempted to say they simply don't count at all. Plus, we can't delude ourselves into thinking that any DirectX-based games would be given any more effort than a quick Cider wrap and boot out the door. It's all that's been happening in Mac gaming since the Intel transition, with the exception of developers like Blizzard and Id who were already developing games cross-platform from the ground up.
OpenGL is a graphics library, which only addresses one portion of what DirectX addresses and from the anecdotal evidence I've seen around the web, DirectX is apparently easier, or more pleasant or something, to program for than OpenGL. The fact that a newer iteration of OGL is being adopted now doesn't change that. The amount of effort needed to rewrite a DirectX-based game engine to use not only a completely different underlying graphics framework, but a whole other set of OS-specific APIs is most likely monumental. As much as the Mac market has grown relative to its old size, it's still very small, making it doubtful whether it's enough to justify that kind of work.
I don't mind rebooting for more serious games (i.e. the HL2 series) because, having played Mac "ports" of them, the speed, graphical performance and hardware support they have on Windows is unmatchable. If Apple gave two shiuhh, hoots about gaming outside of the iPhone OS, this might not be the case, but they've made it painfully clear for the last decade that they don't.
I'm sorry if this makes me a "Debbie Downer". There are many things about OS X that we can be exceedingly proud of. Games aren't one of them.