After upgrading to WordPress 3.0.1 I suddenly came to a point where I could no longer just drag and drop widgets to my sidebar.
The only solution to work with widgets was to got to “Screen Options”, enable “Accessibility Mode” and klick my way through configuration.
Googling for a solution came up with that workaround and “disable all plugins”. Luckily I had not installed too many plugins recently, and the culprit was found quickly: after disabling the Feedburner Widget plugin dragging and dropping works perfectly again. I have no idea what might be wrong with the plugin, but maybe the author can find it.
Hope that helps someone with the same issue…