I do like this hack very much. Have the "pop up" version on my forum.
The only issue it raises, I've noticed, is that it kinda breaks down the integrity of vB's "PM Tracking" system, since recipients can read a PM without confirming receipt. Not a big deal, I know.
Other than that I have to say it's very useful and makes it very easy to clean up a crowded in-box.
Only suggestion I can make for future improvement would be to AJAX-ify it to make everyone happy. Clicking a PM title would displays it's contents inline directly underneath (and confirms "receipt" with tracking system.)
Have a "collapse all" and "expand all" button to do the same en-masse.
I think that would be just about perfect, and might make
everyone happy.