Proof-of-work systems are cool, but this doesn't work, economically.

A proof-of-work system requires a verifiable amount of computation time before you can do something. The idea is that "honest" computers are mostly idle, but that this should slow down spammers.

The problem is that it may not be possible to seriously inconvenience spammers without seriously inconveniencing honest clients, even if both run native code (e.g. hashcash). The article limits the honest guy to some crappy smartphone's interpreted Javascript running an algorithm that is very much optimized for 32-bit machine words (instead of Javascript's floats). If the smartphone can do the work in 10sec (which is annoying enough), the spammer should be able to do it in at most .01sec: enough to send ~8.6 million messages per day.