While Electricity Cost < Block Value, miners will keep adding more and more mining power, be it FPGA or whatever, until Electricity Cost = Block Value. Basically, miners will keep adding mining power as long as it's profitable. So the only thing FPGA boards will do is increase the difficulty, not save on power wasted.
It's not actually "Electricy cost = Block value", it's "Electricity cost + amortized hardware costs = Block value". As hardware becomes more energy efficient more money will be spent buying the hardware and less will be spent buying electricity.
Actually the equation is more like "Electricity cost + amortized hardware costs + time wasted replying to posts on this forum like this one = Block value"
