...There had been a lot of discussion about ultracaps on this blog months back , but it appears that Tesla isn't going to tell us everything they are considering at this point.
I have to imagine that they are considering all the possibilities for ultracaps, but may not have much to say about it until they day that (whatever they do) it is ready to show up in a product. Along with being (as you suggested) a way to accept a charge more quickly when hooking to a charger, they also offer a benefit at being able (theoretically) to capture more regen energy under hard braking that might produce more current than the regular pack could accept (on short notice).
Primary difficulties with ultracaps are wide range of voltages based on stored energy (compared to batteries which have narrow voltage ranges) and low storage densities (which may be on the verge of being solved with upcoming nanotechnologies). Also, the difficulties with quick charge are not just ability to accept charge, but also the need for extremely high current lines which are not typically found in residential areas, and can be quite dangerous...