Apple would need to have it's own game studio for such an Mac exclusive title. Microsoft does it with Flight Simulator, Halo etc., Apple doesn't have anything comparable. I can't see any big game studio making a Mac exclusive unless they are being paid big bucks by Tim Apple.
FUN FACT: Halo was originally going to be a Mac OS X-only game. Microsoft, sensing what could happen and needing a breakaway hit for the XBox, bought Bungie...and now we have the Halo we know today.