Not all PM's will re-appear, only those which exist in the inbox of either (or both) participants, and both participants (regardless of if they deleted the PM or not) will receive the message back in their inbox.
Sadly, this isn't true. I know for sure that both another user and me had deleted PMs we had exchanged before the software update, and they are back again ... so even PMs that were deleted by both participants have reappeared.
By the way, the "Who's online" list at the bottom of the main page doesn't seem to update properly. In the last few days, it has always shown the same users. If you click on "details", you get an updated list, however.
