You'd like to be able to do it automatically from your build process, but you're forced to go run the Apple Jar Bundler and tweak all the settings manually every time you build.
Well no more! JarBundler is a feature-rich Ant task which will create a Mac OS X application bundle from a list of Jar files and a main class name. You can add an Icon resource, set various Mac OS X native look-and-feel bells and whistles, and maintain your application bundles as part of your normal build and release cycle. It is free software licensed under the GNU General Public License.
This release is based on the earlier work of Seth Morabito's JarBundler 1.4 found at http://loomcom.com/jarbundler/