CSS is a pretty damn up2date game, it last updated less than a month ago and now has Mac OS X support.

For the GS on Mac OS thing, there is parallels desktop 5, which really runs all windows applications inside Mac OS, but of course performance of this apps are uncomparable to native windows. f.i. With bootcamp win7(note that this is nearly same thing as win7 on a PC with smiliar specs as iMac) on iMac, you get 512+ fps on single cube model, while with parallels you only get 40 fps.

Imho as for mac support, any other support other then native Mac support is unacceptable.

Editors would require partial rewrite in large chunks to natively run on Mac, also that requires engineers/programmers experienced with Mac. I think engine itself would not be very hard to get it run on mac other than the plugin and the shader system.