This is a very interesting development for visual effects in Blender. It's an addon for roto masking that's available in the latest SVN builds. It's so simple you really don't need a tutorial for it. The demo video below demonstrates it quite well.
Check the latest builds at graphicall.org for builds that include this feature.
Showing of a quick roto mask right inside Blender with my new RotoBezier addon. Thanks to Campbell Barton for all the API features and fixes needed for this :)