This is an idea I had a while ago, which is basically a monetized marketplace for compute cycles. Maybe similar to BOINC but with monetary incentives, not just warm fuzzies for having helped try to cure cancer or something. The tricky part is that neither party can trust the other, and with money involved, there is certainly an incentive to cheat.
The decentralized structure of mining piqued my interest because individuals are essentially expending compute cycles for monetary reward.
The biggest problem with general purpose grid rental, which I never came up with a good solution to, is securing the laborers so that they could be trusted enough. In contrast, mining, by its nature, does not require trust. The block hashes or it doesn't.
As for the bitcoin system, it raised an interesting hypothetical, that is what if such a compute cycle marketplace existed. As a buyer of compute power it would be in competition with mining, and if the profitability of mining is relatively lower, people will jump ship until the difficulty drops and the profitability is on par with selling to the compute cycle marketplace. Because the compute cycle marketplace could conceivably tap a large financial resource, its effect on mining could theoretically be a very strong one, and total network THash/s could drop a lot.
No big deal, that's the law of supply and demand. UNLESS mining consumes a small enough minority of compute cycles, in which case the entire system is vulnerable to undoing transactions.
I doubt this will happen, but it's maybe food for thought.