I've been thinking about this a lot lately too. There are biological systems that exist in vast quantity, like, say, ants or blades of grass (or, simply cells).

I've been thinking about this a lot lately too. There are biological systems that exist in vast quantity, like, say, ants or blades of grass (or, simply cells). I think there is a way to make them computing machines (there was a paper from Adleman, of RSA crypto fame, years ago about it, for example). The challenge is getting the problem broken down across billions of individual compute elements, and of getting the data in and out quickly.