> the problem boils down to how you define the term 'supercomputer'
Here's one attempt: A machine the can run the large challenge problems of the day in a reasonable amount of time.
> they are actually far closer to each other in terms of architecture and
> how they are used
If by, "how they are used" you mean, "programmed with MPI," sure. I can program the multiple cores on my desktop with MPI too. But the architecture of a Jaguar-class system and that of this Raspberry Pi cluster are vastly different. Sure, they have processors, memory, network and I/O but so does every other computer in existence.
Think of it this way. This Raspberry Pi thing is about taking some existing components and hooking them up in an interesting way. Building HPC systems is about designing a SYSTEM from the ground up, tailored to meet the most demanding computing challenges of the day.