this worked for me
I had to do this in this exact order on my machine with the Intellipoint software.
1) Set the center button to Ctrl-Alt-S (or Ctrl-Alt whatever you want) and apply/ok
2) Create a new shortcut of the windows task switcher in the quick launch bar
3) Drag that new shortcut to the desktop. It will be named Windows Switcher(2)
4) Right click on that new shortcut and select properties. Set the short cut key to the same value you set the mouse button to and hit apply.
A previous post mentioned this idea, but it didn't work for me if I didn't do it in this exact order. E.G. just setting a shortcut key to the existing icon in the quick launch bar doesn't work. Setting the shortcut before setting the mouse...