The SL gesture used three fingers; Lion instead overloads two-finger scrolling with back/forward. It's a bad move for UX since it's not immediately obvious what's going to happen when you drag two fingers to the left/right (esp. since Lion also killed scrollbars, leaving no indicator that there's content to be scrolled).
As someone who uses this gesture all the time I'd be very annoyed if a page hijacked the scroll to prevent my forward/back.