his latest blog entry, Scott Lowther describes a unique motor geometry proposed by JPL in 1972. In the conosphere concept, the solid rocket motors in a multi-stage rocket would be nestled in one another. This strategy would optimize the use of space in the rocket. For example, the 2nd stage nozzle would also serve as the froward closure for the 1st stage, and so on. Scott, has a lengthier description and a couple of diagrams, one of which I've cross-posted. I'd like to see the thrust curve for such a motor.
Update: R2K points out that the proposed ARCA Haas vehicle resembles the first step in the evolution from a standard staged rocket to the conosphere. (see the other diagram on the Unwanted Blog post linked above)