OK clearly some creative solutions might be needed in the interim, that's granted.  But, when and if Bitcoin achieves a really wide level of adoption - let's say a Facebook level - and of course the effects of Moore's law - what if the client was re-writen to require everybody that's running it to do a little mining?  I'm not a mathematician but I bet there is one on the forum that can tell if this is viable...  Let's just say Bitcoin has a billion users and each and every one of them dedicates 10-20% of their CPU power for a couple of hours per day - would that be enough to support the network?