Open outlook in "safe mode" and then delete it from the outbox. Is this your work-outlook? If it is, you might have to bite the bullet and call your exchange admin and have him toast it on the server side.
to open outlook in "safe mode" do this:
Click: Start
Click: Run
Input: Outlook /safe
Click: Ok
The other option is to play the waiting game. If it is italicized and you don't have any errors you can just let it sit over lunch or something and it will eventually send and then get returned/undeliverable by the recipient (depending on their local rules).