Hey Bigweb, here's a suggestion for you...
Given how many people there are on here, and the amount of threads being created (many that a person would have no interest in), how about adding a "Mark all threads read" link to the "View Unread Posts" page?
Now that the site's running so much faster, it may not be as big of a problem, but in the past, the time it takes to get from that search to the main page, and then scroll down & click the mark-all-read link, I'm marking some things as read that have been posted since I did the unread posts search - so I never know of their existence unless I specifically go into that category (which I don't often do unless I'm creating a new thread in that category, or looking for a specific post in there as a reference).
Usually when I view new threads for the first time after being away from the site for 8-10hrs or more, there's usually about a page full of threads listed, so I read the ones I want to read, then leave the ones I know I don't want to read, don't need to read, or threads I'm simply not interested in.
Having the mark all threads/forums read link on the view new posts results while not totally erradicating this potential problem, would probably lessen the chances of this happening to others...