I could not find this anywhere on google in one place, but I pieced it together from clues Steve found by poring over a half dozen other blogs.
To get Java 6 to run on Leopard, you must go into
/System/Library/Frameworks/JavaVM.framework/Versions and blow away the
Current and CurrentJDK symlinks. Make them point to A:
$ sudo rm Current CurrentJDK
$ sudo ln -s A Current
$ sudo ln -s A CurrentJDK
Now launch the Java Preferences app and move the JDK order to the way you want it.
Setting the symlinks manually to 1.5 or 1.6 will make text apps work, but
jconsole will blow chunks. It seems like the Java Preferences app changes
“what’s in A” and then the symlinks make that work.
If you have the symlinks set up in the pre-Leopard way, the prefs app won’t fix them.
Sigh. Apple.