I can scroll up (but not down) through threads with the scrollball on a Mac mouse, but the ball has no effect on the main forum page or any sub-forum pages. On those pages, I have to use the scroll bar.
Correction: I can scroll up but not down on any page on the forum. Forum and sub-forum main pages default to the top of the page, so that's why I didn't notice it before. In order to scroll up, I have to first scroll down using the scroll bar.
Addendum: I can also scroll left and right.