The mouse was frozen when I booted up my machine at ork this morning. It's a USB mouse with a serial adapter (silly thinkpad docking station at ork only has 1 USB port and I don't have a hub yet). Rebooted a few times, unplugged it and checked the adapter, swapped adapters, plugged it into the USB port (orked okay). Then, I took a closer look at the icons on the dock.
I'd been plugging the mouse into the keyboard port.
It orks okay now.