I don't think this would work in real life. When Bill Gates was proposing something similar, one of the engineers on the anti-spam team built an FPGA that would

I don't think this would work in real life. When Bill Gates was proposing something similar, one of the engineers on the anti-spam team built an FPGA that would allow a spammer to solve such puzzles quickly enough to prevent spamming. His observation to me was that even though such a technique would work, botnets had eliminated the need for specialized hardware.