Java (at least swing) apps behave oddly, by which I mean they show up as grey boxes. According to the xmonad faq, the problem is that Java uses a hardcoded list of wm's (rather than properly responding to wm events) which does not contain xmonad.

http://www.haskell.org/haskellwiki/Xmonad/Frequently_asked_q...

Even with the suggested fix, Java still has some focus issues.