So first, thanks for that extension... Really awesome !!
Just two think (for now) !!
if a user try to access a private discussion that he doesn't own or participate in, he fall on an error "Whoops, PrettyPageHandler" !! perhaps in that case the user should find a 404 instead !!
And I don't know if you are aware of it, but the first level reply doesn't show up in the discussion list.. I mean the small sentence saying "theDude reply few second..etc", it stay on the "theDude2 started...etc" !! It start to change and show "reply" for the second reply... (get it? sorry for my english)
There are some stuff we can add to that extension... Like as you do for the Profile page, why not a messenger page with all the private discussion (with a link under "view profile") !!??? And also put another link to that new page in the "SessionDropdown Menu" ??
Or just a link to "/?q=is%3Aprivate" ?? (I guess, I can with the "link" extension but, that's not a nice place for a "private link"...)