With all due respect to Guy, this will never work for a simple reason that there is absolutely NO incentive to EVER use mining contracts vs buying bitcoin outright (in OECD countries). With the price of bitcoin approaching the cost of production for marginal players, all profit of such contract will be retained by the cloud mining company. I looked at every single cloud mining operation, and basically it is just a license to lose money (for buyers of such contracts, of course).
In theory, cloud mining has to prove that buying such mining contract will result in the GUARANTEED lower price per BTC acquired than could be obtained by buying bitcoin on the open market. However, such proof does not exist at the moment and is unlikely to exist in the future as well. Case closed.
People say the exact same thing about buying miners and have been saying it since GPU mining. Cloud miners have lower up front costs involved due to not needing to ship all around the world and they generally have cheap electricity decreasing running costs. In that scenario, if cloud mining is unprofitable, then buying miners is even less profitable for the majority of people.
Cloud mining will attract a different breed of people than mining with physical hardware. Cloud miners will be less hardcore and more likely to jump ship at the first whiff of unprofitability. They would probably have no notion of the concept behind p2pool and would have no qualms about pointing their hashes at the most profitable pool even if it were the largest pool with over 51% of the overall hashrate.