Actually, I disagree. Most (all? I'd be interested if you could find one which isn't) forums are arranged like this (and for a reason) - I prefer seeing what the actual question/initial post is and the first responses. Also, if you want to read a whole thread, it would be foolish to have to go to the last page, and then read 'backwards'.
Maybe there could be a setting you could change, or a button saying 'Reverse post order', but definitely not by default.