Modified 3x2 Plate with Rubber Bristle Bottom and snap-on cover? I've had this odd little part laying around my desk for some time - does anyone know what it is?  There's no Lego logo anywhere, and I've searched BrickLink somewhat comprehensively, so I'm guessing it's from a compatible set.

It's a modified 3x2 plate, but without a hole, and the red bottom part is sort of a rubbery, grippy substance.  The "cover" is softer, non-ABS plastic, and can snap onto the plate either way.  

I'm guessing it came from my wife's nephews in a whole pile of stuff they gave us - mostly Lego but there was some MegaBloks stuff mixed in too.  Probably not from after 2000 at the very latest.
So, the curiosity bug bit me again.  The mysterious part appears to be a Kre-O "Motion Brick" which appeared in various sets. In my research I found that there is a variation (hollow vs. solid) in the studs, as shown in the picture.