if this happens, bitcoin would become valueless. The value of bitcoin comes from the fact that you can't just pump them out, the energy requirements required to solve the calculations required for mining are one of the limits to bitcoin production. Free energy like feasible solar or dyson spears would just be a license to print free digital currency making it worthless.
The network recalculates payoffs based on the hashrate. If electricity becomes cheaper, hashrate goes up, and the payoff per hash goes down.
Dyson spheres are not free to construct and the energy they provide is limited.
I knew I couldn't be the only person with this thought. Solar powered Bitcoin mining will eventually be economically viable :).