Differential Hypocycloid
forum thread: http://dev.forums.reprap.org/read.php?14,35968
video of CAD model (by triffid_hunter): http://www.youtube.com/v/bRn1K2XeWVE
blender model: File:Differential hypocycloid.blend
Hopefully this will be a solution for the too-fast-extruder dilemma.