Oh, I didn't realize you were still talking about allowing arbitrary computation. Yes, that would be extremely useful, but it would still be hard to establish as secure.
Pretty much every consumer computer in the world already has restricted-execution environments for running untrusted code from the internet. They're called Java and Javascript, and they're a bountiful source of
security vulnerabilities, not so much because they're sloppily implemented, but because in a hostile context it's extremely difficult to establish whether a given arbitrary computation is safe to run. Do the restricted-execution environments cited in that earlier thread provide better security guarantees than current java and javascript implementations do?
