Sun have started to release early builds of J2SE Mustang (1.6) on their java.net community site.
This will allow developers to get uptospeed with new features before the official GA version is released, and even before the Beta releases.
Currently the site has a “Build 12” release – the release notes look like they cover a lot of bug fixes but no new major features yet.
Although J2SE SDK development is still not Open Source (which some people would like), the site also releases the source code along with the binary builds so you can take a look at the source.