I have done some initial testing and Metro works fine in Mountain Lion. The only problem is Apple. First, in Lion they get rid of the scroll bar arrows and now they won't let anyone install software except through the stupid Apple store. Also Mountain Lion will not install on perfectly good and fast Apple Macintosh computers like the quad core Mac Pro I have. Ironically I have seen it installed on not so fast Windows based PC's using VMWare. I am certainly
not an Apple fan-boy at this point.
To Install Metro in Mountain Lion, you must
turn off gatekeeper.
Also there is a bug in Metro 6.5.2 that makes some of Apple's Cocoa based plug-ins display wrong. This occurs in Lion as well. This problem is fixed in Metro 6.7 (unreleased as of yet).