We often install xcode in the 'Developer' folder. when you uninstall xcode, you can open a PDF file named 'About Xcode and iPhone SDK.pdf' and located in 'Developer' folder.
you will find a command in this file
$ sudo <Xcode>/Library/uninstll-devtools --mode=all
if you install XCode following its default settings, XCode will be installed in the 'Developer' directory, and the command for you is
$ sudo /Developer/Library/uninstall-devtools --mode=all
I found it missed one file during the uninstallation process. I also dont know the reason until now.
Note: You should manually delete the files in 'Developer' directory after uninstallation, otherwise you will encounter with some unexpected errors.