I have recently investigated the integration of the Nimbus look and feel. I have committed the Nimbus LaF support to the IzPack Subversion trunk using the latest Nimbus weekly build.
If you launch an installer, it looks pretty good:
… but you will not be able to go much further as it crashes 
I have tested this on Mac OS X, Linux and Windows with Java 6 with exactly the same problem each time. By the way, Nimbus won’t work (yet) with Java 5 (and of course even less with Java 1.4).
I have reported the issue to the Nimbus project and I hope that they’ll be able to either give workarounds or fix the problem (Romain, are you there? 
).