Comment 21 for bug 984603

I spoke too soon - the backported driver updates certainly helped, but I have now realised that Skype was only working with an external analogue microphone. When asked to use the webcam's built-in microphone, I was back to freezing images - unless I defocussed the images, in which case it worked! Some bandwidth issues then .... Hans was once again very helpful, and correctly suggested

   This sounds like your setup has an usb-2.0 hub between the
   camera and the root-hub (*). This means that you either have a
   intel sandybridge machihine, where such a hub is integrated
   into the chipset, or you have put a hub in between yourself.

   *) Unfortunately the linux EHCI scheduling code deals poorly
   with multiple isoc stream to a USB-1 device attached through a
   USB-2 hub.

This was indeed the problem - I have a Sandybridge chipset. Apparently there are no known workarounds (apart from making sure your webcam is not a USB-1 device, or using an external microphone). My best guess is that the bug reports relating to "skype only, other apps work fine" are probably better labelled as "combined video and audio from the same USB-1 device only, video only apps work fine".