Comment 4 for bug 277195

Confirmed on Jaunty.
This is a bug with Viewport Switcher plugin. Disabling that plugin will stop mouse scroll workspaces - on desktop or otherwise.

Reproduce by enabling Viewport Switcher and setting Move Prev and Move Next to
Mouse Button 4 and 5.
 Try mouse scroll above the search box, near or on the 'Filter' label in CCSM
 This is annoying; the expected action is to do nothing in this situation and
to scroll workspaces only if hovering over the Desktop.

This bug effects at least the Banshee movie canvas, the Typing Break lock screen, the status bars at the bottom of most applications, and often on padding between GTK widgets.