A few times, I've had a problem where the progress bar would stop progressing when I tried to empty the OS X Trash. Even going into the Terminal and trying to delete the files via sudo rm causes that command to just sit there, making no progress. Rebooting the computer doesn't help. (And, just to be even more fun, the computer may not even allow itself to be rebooted without restarting the Finder first.)
Every time this has happened that I can recall, the problem has been with the "ownership" of some files in the Trash.
Luckily, there's a very easy solution. Go to the Terminal, and enter:
sudo chown -R xxxxx ~/.Trash/*
(replacing xxxxx with your usual OS X "short" login name.) You'll have to enter an admin password.
After that, you should be able to Empty Trash from the Finder as usual with no problems.