Not that I'm an expert on Outlook or anything, but this helped a friend of mine and I thought I'd pass it along. If you have a better way of troubleshooting this, feel free to leave a comment!
OK, so this is what happened: my friend tried to send an email with an attachment without realizing that it was a file over 100 MB. She got impatient and terminated it, and now every time she opened Outlook again it seemed to hang.
The fix was to unplug her network cable for a second, launch Outlook, and delete the message. With no connectivity, Outlook left the message alone, and she had no problems deleting it. I'm sure there are other ways to go offline, but I focused on a quick fix here.
Another great example of the flexibility you get from queuing in action. Hmm, perhaps I should spend more time reading up on Windows Workflow Foundation...