How about "up" and "down" buttons on the thread pages? At least on the top and bottom of each thread page. Going to the bottom to read new posts in an old thread would be nice and a "back to top" button would be sweet at the bottom of each post.
Since you've made each thread page display 50 posts, once you get to the bottom, it's a long scroll back to the top to get to the selection bar to read another thread.
Since you've made each thread page display 50 posts, once you get to the bottom, it's a long scroll back to the top to get to the selection bar to read another thread.