The stopping of spam has more to do with the spamming scripts not having javascript interpreters and not being able to do any calculations at all rather than the artificial time penalty. The most beneficial implementation of PoW is with a lightning fast calculation that won't even be noticed by the human visitor.
Agreed. That's how wp-hashcash works. If/when the spammers get around to integrating a javascript engine, we'll have to see what happens. So far, I've not had a single spam comment get through.
As I said above, no javascript interpreter is needed for the spammer. You can calculate sha-1 hashes in any language. It's more about proving that you took the cpu cycles to do it. Unfortunately, the server must have to do the exact same work to compare answers!
I'd say it's probably impossible for a server to verify if a client has a working javascript interpreter. I imagine you haven't gotten any spam comments yet because (a) not enough sites use this system yet to make it worth someone's while to write the code to bypass it, and (b) the time delay required to do the calculations. Once widespread use of this system reaches a certain critical mass you'll probably start getting spam.
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