But all is not lost.
(With Outlook 2007)
If you go into your sent items, open up the offending email, and go into Actions | Recall this Message, outlook will attempt to delete the message from the recipient's inbox before he has a chance to read it.
Figure: Actions | Recall this Message (Outlook 2007) (With Outlook 2010)
If you go into your sent items, open up the offending email, and go into file | Info | Resend Or Recall | Recall this message, outlook will attempt to delete the message from the recipient's inbox before he has a chance to read it.
Figure: File | Info | Resend Or Recall | Recall This Message (Outlook 2010)
Outlook will tell you whether it was successful or not