After "Mark all Topics as Read" is clicked the lower part of the screen is always empty. How about doing an implicit "Switch Board" at this point to bring up the board list automatically? Would this also require for consistency that when clicking "Switch Boards" and clicking on a board with no new posts the board list also be brought up automatically? Too confusing?
Efficiency in switching from Unread Posts in one board to another will be less important once the user can specify which boards he would like to see in the default view.