Civ 4 has had problems with the 8600M GT ever since the 8600's introduction in the MBPs - Aspyr's stance (which, to my knowledge, has not changed) is simply that they 'don't support the 8600M GT at this time'. Of course, an alternative is to buy the Windows version + Warlords for the same price as the Mac vanilla C4 (which also gives you the option of Beyond the Sword for $20 more), which runs much better on the same hardware than the Mac version.
(yeah, I'm bitter)