This sounds really interesting. I almost want to think up some real world problems just to see if I could build their solutions within EOS. :)

Will there be a testnet for people who want to play around with this?

Once a block is deemed irreversible validate and precondition never have to execute again whereas apply must be executed to generate the deterministic state every time the blockchain is synchronized.

I really like that. Seems like an efficient way to go if much of the code to validate things has already been run before it was applied and won't need to be run again.