This likely won't happen until it costs about as much in electricity to mine a Bitcoin as it does to buy it. As long as mining appears to be cheaper people will pump money into mining equipment (and to pre-orderers not using a calculator with high enough difficulty adjustments it will look very profitable until most people's asics get delivered). When mining finally reaches the point of being barely profitable at all even with already owned asics it'll be a very good thing for the network. We'll have reached a point that will make it extremely expensive to 51% the Bitcoin network so it will be much more secure.