Pygame will soon have support for cameras and some computer vision functions. This is likely the easiest way to integrate vision with Activities.
While still under heavy development, source for the branch of pygame with camera support and vision functions can be checked out from: 
Currently, the only game taking advantage of vision processing is Colors!, which allows you to use the XO's bright green charger as a virtual paint brush. It accomplishes this by converting the colorspace of a frame from a gstreamer buffer to HSV and finding the centroid of the specific hue of the charger. This is a fairly complicated process that will soon be made much simpler upon completion of camera support for Pygame, a GSoC 2008 project.
See Living Pointillism.