I just made it work. For windows:
1) Uninstall the jre you installed.
2) if you like (I like, personally) remove your mozilla install. Reinstall it afterwards.
3) Install the sun jre plugin.
4) completely kill mozilla (including the quickstart)
5) copy the NP*.dll files from the jre bin directory (C:\Program Files\JavaSoft\JRE\1.3.1\bin) to the plugins directory of mozilla (C:\Program Files\mozilla.org\Mozilla\Plugins"
6) start mozilla.
Yeah, it should be better documeted somewhere.