If you haven’t had a chance to read it, or just want an easy-to-access copy, here is a pdf Ray’s magnum opus.
The topic came up at a late dinner with David the other day; we were explaining the potential computational power within an ordinary rock to his girlfriend.
I’m sure most other women would have certainly appreciated that conversation.
Also, if you have any kind of advanced mathematical background, you may enjoy the short academic paper Kurzweil cites in his section on the limits of computation and reversible computing: Universal Limits on Computation.