They tend to overheat past that point in normal computers. Enthusiasts can get production chips running at up to 6GHz, but at that point they're having to use things like liquid nitrogen pumps. A passive radiator with an air fan can only conduct away so much heat. My newest PC (A core i7 or something, I forget exactly what now) has a heatsink that's bigger than my fist.
I don't think the reason to use LN2 (liquid nitrogen) in extreme overclocking is because of the added heat generation. I bet water cooling (which is very "mainstream" compared to the more extreme solutions) could easily handle the extra heat. Afaik you actually need to reach very low temperatures to get the silicon work properly at the high speeds (I guess it has something to do with thermal noise and conductivity, but that's just guessing).
And that is imho a sign that it's not just a power consumption / clockspeed tradeoff, but instead there are actually limits on how high you can go at room temperature.
