Bump. I can also confirm the problem exists after installing 2.8.4 (and deleting old source files during the install process)
I was able to solve this problem on my own. It appeared that something had become corrupted in the database during upgrading versions of WordPress; most likely the wp_options table. I have seen many posts advising which database fields to edit and also a plugin which resets the widgets but none of these fixes worked for me. In the end I resolved the problem by importing my post, user and plugins data tables into a clean database from a fresh install (leaving the wp_options table) and exporting the SQL for import into the blog – this meant having to reconfigure some of the options that were originally stored in the wp_options table, but now the widgets behave as they should.