I don't think I've used the search function for the board before tonight - I've really got to say, it's quite nice how you have it set up. Very easy to find what you're looking for and get to the proper thread.
Is there any particular reason your current IP address is listed in the header of each message, though? I thought it was odd that someone had the same IP as I did, and then I noticed all the posts did.
It has to be the most recent post in the forum to do it. So, if you are reading from the bottom up, and using the next thread option to navigate, when there is not a next thread to go to you get the mark forum read option.