Some archive files (typically .tar.gz created on other systems), when decompressed into folders, will then be untrashable. An example is the GNU LibObjects distribution (not from Darwin). When you try to empty its folder from the Trash, you get -43 errors.
I have created a little droplet app to address this - DropObliter8 - available from my iTools disk at http://homepage.mac.com/howardoakley/.
Mac OS X Hints