Some bods have been working at porting over the FreeBSD version of JDK 1.6 to OS X. As you might expect it usees X11 rather than native swing bindings; but that is sufficient for a lot of service side stuff. Hello I spend a lot of my day running X11 apps anyhow. Also of interest is that they have this running on 10.4 and 10.5 which the apple version of the JDK certainly wont.
Take a look at Landon Fuller's site for more information. A bit more to do on hotsport stability; but this is a valuable resource when you just gotta have the latest and greatest JDK on your mac.