webcam works on one of two Lucid systems

[Adam Funk]

I have a Logitech USB webcam (USB ID 046d:0804) that works
effortlessly on one computer but not the other (both running Lucid).

On both machines I get nearly identical entries in the syslog when I
plug it in (with varations in the numbers according to the USB port).

usb 2-4: new high speed USB device using ehci_hcd and address 2
usb 2-4: configuration #1 chosen from 1 choice
Linux video capture interface: v2.00
uvcvideo: Found UVC 1.00 device <unnamed> (046d:0804)
input: UVC Camera (046d:0804) as /devices/pci0000:00/0000:00:04.1/usb2/2-4/2-4:1.0/input/input7
usbcore: registered new interface driver uvcvideo
USB Video Class driver (v0.1.0)
usbcore: registered new interface driver snd-usb-audio

On both machines /dev/video0 appears, and it shows up in Skype's
pulldown menu under video options.  But on one machine the video from
it is always completely black.  (I've tried cheese with the same
results.)

Any suggestions for debugging?