If you are having troubles deleting files from Trash, use XRay, a shareware app. It allows you to change authorization without using arcane commands in Terminal.
I was so frustrated by the undeleted file as it was interfering with other files in Trash so they could not be deleted either. XRAy is a wonderful product and well worth the $10 shareware fee.
[Editor's note: XRay has been mentioned here before, but not in the specific context of dealing with undeletable files. I actually worked with the author on other means of deleting the troublesome file, and nothing we tried was successful -- not that I know all the tricks, but the ones I did know didn't do anything to help the author.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020528082554325