Yes, this is definitely an interesting question. Salvatore says above that it's an order of magnitude, but it would be great to see the numbers for some specific CPU.

I'd also love to see how much of a penalty is imposed by the OS for things like virtual memory support. It would be fun to see the numbers for something closer to bare metal like LoperOS, or TinyOS without Xen and running directly. Not that this would be practical for many real world circumstances, but I don't really have a clue what the overhead is.