. . . For #2, I imagine that some do use GPUs, as people are very competitive. #3 is hard. If you're getting paid, someone is paying, so we are mostly looking at commercial applications. But these tend to be sensitive or secret, not suitable for wide sharing. For example, an obvious application would be rendering graphics for a new movie. But the producers wouldn't want their scenes to be leaked.
To solve these problems and then focus only on people who want to receive bitcoins would be doubly difficult.
I don't know if the sensitive or secret aspect would be of too much concern in most applications, even those secret or sensitive because there are ways of finding keys that do not reveal the data secured with the keys. For example, many people use TrueCrypt and encrypt their whole hard drives. I believe your chosen password unlocks a file, which is itself the actual key to the data on the hard drive. Simply providing people with that small file and then assigning them a portion of possible passwords to check through would provide nothing but the password, which they could do nothing with. Likewise when someone would provide the handshake file for a WPA protected wireless system they wanted to get into. I forget what this is called, but much of crypto works this way, where a password unlocks data that unlocks something else. I would think it would be possible to take a small portion of the Wikileaks file and brute force the password without giving away the whole file. Even in the case of rendering a movie, seems that the possibility of the rendering in one movie clip, or one portion of one movie clip, might not be all too risky for the producer, if it was released. And, the part about accepting Bitcoins was not really central to the idea except that people who are already generating bitcoins with GPU's might be inclined to accept Bitcoins. As a Bitcoin payment can be exchanged for dollars, I thought payment for CPU time might be a convenient way of accepting payment. I suppose PayPal could work almost as well, but there are not too many other options for conveniently accepting small payments online without wasting alot of time and being eaten alive by transaction fees.
Draaglom's concerns seem a bit more daunting, but cheating has been resolved with Bitcoin mining (at least insofar as the mining host not having to trust the GPU clients) though the proof of work hashes sent back to the host, as well as the 'reward' being signed so the client can't cash it for himself. Here's a person that has software to crack "lost passwords" and uses GPU's to do it ... http://www.insidepro.com/eng/egb.shtml
. Seems to me that there may be cases where even with a $500 + ATI 5790 like mine, a password that might take a few years to crack on my own might only take days if I were tapped into a network of hundred of other like computers. I just don't know how efficient the whole plan would be. Anyone care to calculate how many $500 dual GPU 5790's it would take to rival the Chinese Tianhe-1A (which reportedly cost "billions") or the U.S. Cray Jauguar in terms of a petaflop/dollar basis? Seems like the supercomputers would have a limited lifespan before more billions needed to be spent when the next generation of fast processors came out the following month. Gamers, on the other hand, will always have the fastest thing out there at any given time.