I have a Unibody Macbook Pro with the latest version of Snow Leopard. I often use the laptop with an external mouse, monitor, and keyboard. I have had this configuration for a few years and only in the recent months have had a mouse issue.
The mouse seems to "half" work in that navigation and clicking works. But things that rely on hovering do not. So expose, highlighting menu items, etc don't work. For example if I click the Apple logo (top left) and then hover over About This Mac, the menu item will not be highlighted.
I've tried plugging in the USB devices in multiple configurations but nothing seems to cure it forever. It comes and goes. To add to the frustration, if I open the laptop and use the trackpad, the same symtoms will occur but as soon as I unplug the USB port, the track pad (and cursor) begin functioning normally!