I spent a little play time this afternoon writing two simple SWT Java standalone applications. It took about 15 minutes to get Eclipse set up properly (a little annoying), but once I corrected my mistakes, it was easy to build and run SWT standalone applications.
I don't have time for this today, but I would very much like to try out using GNU gcj + SWT to build native applications. One problem I have with this idea is that the GNU gcj libraries are huge, so I wonder how large a simple standalone application that is statically linked would be. I would settle for 5 MB or less :-)
On OS X, the SWT runtime JAR and JNILIB files total about 1.2 MB - not bad.
There are two good articles on using GCJ and SWT (search for "Kirk Vogen gcj SWT" to find them).