As Og stated, heat is a major consideration. There's also a strong likelihood that first-gen ASICs will barely cover power costs by the end of the year.

The only realistic option to sustain long-term operation is to move to second-gen hardware. Despite the PR disaster BFL is, the fact remains that they have delivered, and early orders do reasonably well. The FPGA history also shows they will probably move smoothly with the second round.

Whether 28nm hardware pays itself off remains to be seen, but again - it's the only viable choice.