From my experience with testing ads, I received a good click through rate when I displayed messages in the actual members mailbox message. Mail is probably one of the most active site features, so it'd be great to have an additional ad bind section for the members mailbox message that would be displayed at the bottom of each message when you either create a new message or read a reply. You could probably limit the space to 468x60 ads or create text ads. I tested both and it seems to work.

Example
Author: Name
Date: 22:54 Jul 29, 2008

Subject: Re: Hello
The users message. Sample message here


___________________________________________
^{sponsored ad}
          [YOU ADVERTISEMENT HERE]

Agreed and added to Skadate7