I've got tmux to scroll up, with the mouse wheel, using mouse-mode on in tmux.conf (here's my entire conf).
But I've always had a problem scrolling back down again. Once the cursor hits the bottom of the screen, it immediately scrolls back to the bottom of the whole buffer, where the prompt is. It's not possible to scroll down a few lines in the same way I can scroll up a few lines. In other words, the whole middle section is only visible on the way up.
I just upgraded from tmux 1.6 to the new tmux 1.8 (compiled from source rather than Homebrew, in case that matters), and it's working again now.