1. Install v4l2loopback Linux kernel module (on ArchLinux it is the aur/v4l2loopback-git package) and do sudo modprobe v4l2loopback.
  2. Run gst-launch-0.10 ximagesrc use-damage=0 ! ffmpegcolorspace ! "video/x-raw-yuv,format=(fourcc)YUY2" ! v4l2sink device=/dev/video1 to capture your desktop into the /dev/video1 loopback Video4Linux2 device.
  3. Run gst-launch-0.10 v4l2src device=/dev/video0 ! xvimagesink to show the webcam window.
  4. Run Skype and set the video device to /dev/video1 or run gst-launch-0.10 v4l2src device=/dev/video1 ! xvimagesink to test how it would look in Skype.