There seems to be a fair number of Mac compatible poker rooms.
http://www.mac-poker.net lists FullTilt, Absolute, PokerRoom, Pacific, EuroPoker, HoldemPoker, & Party... on top of those I know Bugsys Club and a few other tiny rooms are Mac compatible!
Oh, and I should also mention that a large percentage of Mac users simply use emulator software so that they can run Windows software, which means that a large chunk of this small demographic don't even require custom software (which is why alot of poker rooms are not targetting Mac users)