William A. Gilbert has released JarBundler 1.6 (forked from Seth J. Morabito's JarBundler 1.4) an Ant task for creating Mac OS X application bundles. If you're using Ant to build Mac applications, you need this task.
I'll be talking about it (and many related subjects) in my session on Mac Development with Java at Software Development 2006 next week.
JarBundler is published under the GPL.