I imagine that heat is going to end up being our limiting factor. It quite possibly plays a big role even now. If the chip is sucking down a max of 140W in worst-case at 2.6GHz or 125W at 2.5GHz, imagine what our chips at 3GHz+ are pulling (worst-case, once again). A CPU's efficiency is near 0%, after all. A CPU produces no light, no sound, no locomotion, etc. It almost all escapes as heat. That's a lot of heat to contend with, as we're all well-aware of right now.