Im seeing an increase of about 50mh/s on average going from Lion to Mountain Lion with a stock 5870
Went from 245 (pitiful for a 5870) to 302mh/s
Still not fantastic, but nice on stock for OSX.
Just a heads up I guess.
Thats almost as fast as what a 5870 should be without bfi_int, so OSX is slowly catching up to the real world.