Now if you want to get really hard-core Apple, how about a JNI example to make a REALbasic plugin? This question has come up on both the Apple Java and REALbasic Network User Group mailing lists, and the answers are usually pretty server-side centric (use SOAP, XML-RPC, etc). I've also played with what's essentially a "cmd-line pipe" from REALbasic, calling Java and getting values from System.out, but that's an ugly kludge.
Java is a great, highly structured way to create headless (ie, GUIless; Swing flames off, please!) logic xplat. REALbasic is a great way to quickly make xplat GUIs. But right now, even on the most obvious platform to tie the two together, nobody's yet shown how to get them tied together natively.