Thanks for the response, Eva.
My problem is not the cookie timeout.
It's that the behaviour I describe only occurs on my board! (Hmmm...just tried yours and it's the same "problem")
Here and on most of the other Vb boards that I've tried, if I view a thread (icon:

) and then click Forum Jump GO (to go back to the thread list), the thread I just viewed shows as "No New Posts" (icon becomes

).
(BTW, I force refresh on every page just to make sure it's not my cache, but it makes no difference)