I've had a nice go at making private messaging capabilities for flockNote work a lot nicer than the out-of-the-box Privatemsg module experience, by simplifying everything to the point that it's closer to the Facebook Direct Message system than the normal Privatemsg UX. (Privatemsg is the premiere way of handling private messaging in Drupal. It's already awesome out of the box... just needed a bit more help for our particular site ;-).
One thing I had wanted to do for a while is prefill the subject field of certain messages. I already have the new private message page appear inside an overlay popup after a user clicks on a link to send a private message to another user on the site.