I think PM system has been running the whole time, but currently you need to click the person's name on the post, then on the profile page select "send pm". Just sent you a test PM.
Maybe we'd need back a little button to send PM's, "send PM" etc, after an image? Profile page is a few clicks away. Might do that, just at the moment we need to drop the amount of all the buttons, functions and text on all topic pages to make the threads more easily scannable (=readable).
But you gave me an idea - why not turn off the PM notification? That would even make PMs useful!