Today I found two bugs which led to the corruptions with Nimbus.
It looks quite well now, the visual differences are because I am running it on the OpenJDK7 codebase, maybe synth has not been updated in JDK7 to work with the ned Nimbus releases:
The first bug was when drawing images with transformations. The solution works (kind of) but is in my opinion very sub-optimal.
I am also fighting with something which could be an xorg-bug, I am not sure ... I prefer to blame my code before others ;)