I went to the installed page of the plugins preferences panel, and disabled a number of plugins I don't need, as well as enabling a non-bundled plugin of my own.

Then (without clicking Apply) I went to the Available page, and downloaded an update for 2 other non-bundled plugins. (This used to be available on the Intalled page too)

I then went back to the Installed page, and all my changes there were reverted.

Environment: Mac OS X 10.5.5, Java 1.5.0_16

Issue was resolved