Mac + Orbiter: BootCamp
On my MacBook Pro, I have set up a 5GB BootCamp partition and installed Windows XP SP3, with hardly any applications. On an external drive (USB connection, though that doesn't matter) I have a number of Orbiter installation folders (different combinations of add-ons, etc.). I boot into Windows, and launch Orbiter. Everything just works, with high performance as you would expect. The only downside is logging out of Snow Leopard to boot into Windows.
---------- Post added at 02:15 PM ---------- Previous post was at 02:05 PM ----------
Aside from BootCamp (which runs Windows natively), virtualization allows you to run Orbiter on Windows in a virtual machine alongside other programs (Mac, Windows, Linux, whatever). This can be more convenient since you don't have to exit Mac OS X, but there are problems. Performance is not very good (I'm lucky if I get 30 FPS); this is probably due to drivers in Parallels Desktop that don't take full advantage of the underlying graphics hardware. I also haven't been able to get sound working. I've tried VMWare Fusion and VirtualBox, but haven't been able to get Orbiter to run in either.