Summary: If you delete the trash folder (~/.Trash) and then recreate it, you must restart Finder before you can drag things to the Trash icon again.
Background/Explanation: I've been doing some disk housekeeping: deleting files, moving them around on external drives, etc. I somehow ended up with a copy of the Desktop DB file in the Trash. The file wasn't visible, but whenever I emptied the Trash, I got the annoying message The operation cannot be completed because the item Desktop DB is in use."
After clicking Continue, the Trash icon reverted to its apparently empty state, but the next time I put something in it and went to delete it, the same message appeared. I dropped to Terminal and thought I would just delete the file there, but it wasn't there. So I simply ran:
$ rm -rf ~/.Trash/
mkdir ~/.Trash/
Thinking that would solve the problem, and it did.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061216221914302