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.
I must agree with BrewGuy (SHOCK OF SHOCKS!!!) I too like Robert Ortega's Lines....I just started looking at them starting with the PPV stuff....it's AWESOME!! Loosie. Your friendly nieghbourhood Canadian. The SWF: