Hi
I run Leopard on a MacBook and whenever I tried to use the middle mouse button it activated 'Spaces'. To resolve this I selected Spaces and held down the mouse button to select Spaces preferences where you will find two option boxes to activate 'Spaces' i.e. 'F8' and, over to the right, Secondary Mouse Button, Middle Mouse Button or, '-'. By selecting '-' it freed-up the Middle Mouse Button for use in SU
Hope this works for you π
Stu.C