Found a great little tool for generating UML Class diagrams from existing Java classes. It's a plugin in Eclipse that outputs the image to Omnigraffle. The good news is that Mac OS X users can create diagrams in their favorite tool.
Its available in the Eclipse Market place as 'javatoomnigraffle' or you can check out the page here.
Granted you have to pay some bucks for Omnigraffle, but its worth it and now with the Eclipse integration, it will make for some good pictures for upcoming blog posts.