I think scarcity is a bit of a red herring. The key is that it has to be more lucrative to play fairly than to cheat (well, it also has to resist attacks that

I think scarcity is a bit of a red herring. The key is that it has to be more lucrative to play fairly than to cheat (well, it also has to resist attacks that are simply trying to destroy the service). Proof of work by itself is not enough. It's the scaling of the difficulty to the potential gain -- the implication being that more people mining == more money to be gained by cheating. The other thing that is compelling about Bitcoin's proof of work protocol is the ideal that every participant is equal. The whole point is to avoid the circumstance where more money means more control. Now, I think we can probably all agree that this didn't pan out -- whoever controls the big mining pools controls the system. I think that if anyone is going to go to the next level they probably have to step back and look at the problem with fresh eyes. Substituting X into "Proof of X" is unlikely to provide the solution, IMHO.