Kirk, everything else works fine, renaming, tagging, rating etc. I first noticed the problem when I tagged a lot of images for deletion, selected them and deleted them. They eventually disappeared from the contact sheet, but reappeared on reopening PM5.
But the problem exists with tagged and untagged, single image and multiple images.
When I delete from PM5 there is no movement at all on the image in the Finder, and it can still be opened and edited via the Finder.
The strange thing is that PM5 works correctly on my August 2011 MacBook Air, also on Mountain Lion.