This is a screenshot of what it would look like - this Quick PM Reply box will be located under the PM message that you have received and are reading.
The username, and subject will be automatically added. The PM message that you are reading will automatically be included in the textarea in quotes.
New version screenshot - you can now see the checkboxes that you can tick to automatically delete the original PM when you send the reply, or to enable URL, signature, save to sent folder or message tracking.
EDIT: This hack has been installed in vB.org. Just try it out here.
Screenshot: