I wouldn't think that an installer is really necessary. A double-clickable application that has the menus and command keys (and a few other little things) like one would expect on a Mac would be nice, though.
The
http://faq.javaranch.com/java/MacOsxFaq links to several relevant resources, particularly the OSXAdapter and AppleJavaExtension libraries, and the
Tailoring Java Applications for Mac OS X and
Make Your Swing App Go Native articles. Of course, to make use of all this, one really needs to have a Mac for testing.
Another thing to keep in mind is that Java 6 isn't widely used on the Mac (yet), so the app needs to be runnable under Java 5.
I've done this kind of thing with several GUI apps; if you're serious about contracting out the Mac polishing, contact me offline (my email is on my profile page). I wouldn't expect it to be a big task.