Where can i get information on writing JNI apps on Mac OS X?

There is a Q&A on building JNI libraries with Project Builder at: http://developer.apple.com/qa/java/java28.html

There is a simple sample code project at: http://developer.apple.com/samplecode/Sample_Code/Java/JNISample.htm

And the release notes detail how to compile the library from the command line: http://developer.apple.com/techpubs/macosx/ReleaseNotes/JavaGMWebReleaseNotes.html