Even though the idea sounds interesting, what about the environmental implications? I mean, SETI searches are far fetched but at least they're good for *something*, this seems like (indirectly) burning coal for money. I don't know how many people are going to be doing this, but if a million people were using two cores a day to do this... what does that use? Let's use a conservative estimate of 50W. 50W * 1M = 50 MW of power down the drain, or 10% of the yield of one of a typical coal power plant. Add another 20% if somebody releases an easy-to-install client that uses GPU power.
What I would like to know is: what is the number of block solving clients that are needed to keep this system running, how does it scale? If only 50.000 clients could help 500M people pay securely with this kind of money, it's fine by me. If 500M people need to be block solving all day, what a waste!