The Freenet Project (external link) has had several long-running e-mail threads over the years on similar schemes. The first was "Hash Cash", where a computer would have to do a certain ammount of work to access a resource. The problem is that Moore's Law works against you, and it could lead to a seperation between those who can afford fast computers and those who can't. The next idea was "Think Cash", where there is a test that a human can pass easily but a computer can't. After all the discussion, it was stamped as impossible in practice. Reasons:

• Suggested schemes often hurt usability for certain users (such as blind people or non-native English speakers)
• Though not yet available publically, the schemes were often already being solved by computers in various university AI departments
• If a harvester really wanted to, they could pay a batch of minimum-wage workers to fill out these forms all day with an acceptable rate of acesses per day