Can PC Call Of Duty: Modern Warfare 3 servers interact with Mac servers so I can play with people who have Macs and not PCs?
According to this FAQ about the Mac version of Modern Warfare 3:
Do Call of Duty: Modern Warfare 2 and Call of Duty: Modern Warfare 3 for Mac feature multiplayer? What about cross-platform multiplayer?
Cross-platform multiplayer, which allows Mac players to play with/against Windows PC players, is available for competitive multiplayer mode only at this time and is not available in Spec Ops mode.
So it would seem that PC players can play against Mac players, but can't co-op together.