Was toying with another idea for close integration with Chipmunk. The gist of it is that Sprite would have a Chipmunk Shape where it currently has a Rect. It could be used by itself in that way, but there would also be a new class, tentatively called Cluster, which would be a collection of one or more Sprites. Cluster would be a wrapper around Chipmunk's Body class, allowing the Cluster's sprites to move and bounce according to the physics simulation.
Anyway, that's one possible way of laying out how Shapes and Bodies relate to Sprites, but not necessarily the direction I'll go in the end.
As well as that little bit of thinking work, I managed to while away several hours implementing a transformation matrix class that works with Chipmunk's vector class. I'm not sure I'll actually want or need it in the end, but it was good to take a break from the thinking, and actually write some code.