The problem is that the cost of a hash is several orders of magnitude more expensive on a mobile phone, than when using a specialized chip. Money, however, cost

The problem is that the cost of a hash is several orders of magnitude more expensive on a mobile phone, than when using a specialized chip. Money, however, costs the same to everyone. I predict that if hashcash ever becomes widely used, people will start trading hashes for money. Ie. instead of wasting 30 seconds of battery power to create a hashcash header on your mobile phone, you'd pay 1 cent to a service that does it for it. And this is exactly what Bitcoin does already.