More Google-fodder for folks who might have the same problem. The keyboard and trackpad ‘disappearing’ under OS X 10.5.1 causes the following to appear in /var/log/system.log:
Dec 16 12:23:01 trixie kernel: USBF: 31305.811 [0x3fa0000] The IOUSBFamily is having trouble enumerating a USB device that has been plugged in. It will keep retrying. (Port 2 of hub @ location: 0x1d000000)
Dec 16 12:23:05 trixie kernel: USBF: 31309.651 [0x3fa0000] The IOUSBFamily was not able to enumerate a device.
Dec 16 12:23:50 trixie com.apple.launchd (com.apple.dyld): Throttling respawn: Will start in 60 seconds
Seems that it ‘forgets’ the internal USB hub for the keyboard and mouse. The external keyboard/mouse work fine.