I did that for the about the site sub forum. Seemed to work OK though the sub forum about this site has stayed unread even though i have read this topic which now reflects no unread items in this sub forum for me. It also marks all the threads as read for me where I have definitely not read them all. In addition I don't think it goes to the last unread it generally goes to the end of the thread it doesn't seem to remember where i left the thread. I know it should work as XDA forums for me works as it should.