Interesting post proposing a workable solution against spam and DDoS attacks. Again, this is stemmed from Bitcoin and the approach is not so different from what was proposed in this article on crowdfunding.
The brilliant idea behind Hashcash is to replace a monetary postage fee with a computational postage fee. In order to send an email, the sender first has to solve a math problem. Legitimate activities suffer an indiscernible delay, but illegitimate activities that require massive volume are hobbled.