I found the problem.
If you're using a plugin or widget that must be installed in the folder of the old widget plugin, that plugin will break everything.
Now that WP 2.2 has widgets built-in there's no need for the old widgets plugin. However, a few plugins/widgets are built to be installed in the same directory as the "widgets.php" file. Now that that file has been moved out of the plugins folder, problems arise.
Your problem plugins may differ, but I broke my blog with the King Text and King Links plugins/widgets. They must reside in the old widgets folder in the plugins folder. Now that widgets are incorporated in WP 2.2 and no longer a plugin, those two break and will take your blog down with them.
Fix: Copy your old widgets folder to your local hard drive as backup. Delete the widgets folder from your blog's server. (If you haven'y upgraded your database, do so.) Reload your dashboard. Now you should be fine.
Sadly, the King widgets series (which powers my entire sidebar) needs an update to work with 2.2 from what I can see. No updates exist at the time of this writing. Check the site here: