I saw this one video on youtube.com(http://www.youtube.com/watch?v=rBq2BnXg1HM) where they were cooling an AMD Phenom with liquid helium and they were able to get it to -250 C but only overclocked it to 6.5 GHz. Why didn't they overclock it more, with that kind of cooling couldn't they get it to 20 GHz without a problem?
You have to know that overclocking isn't just "that simple". Overclocking the CPU too much leads to overall system instability since they didn't overclock the NB, SB and DAC's. Usually the South-Bridge and the DACs cannot function under the temps of -20C.
Also, the CPU isn't meant for getting past 7GHz, some less stable CPU's gets downclocked because they can't maintain a stable system at the designed clock, and they sell it with an different name at a lower price.
A good example is the Phenom II and the Athlon II, they are basically the same CPU, but if the L3 Cache of an "Phenom II" has a defunct, they disable it and sell it under the name Athlon II"
If they were at -250'C then it was probably liquid hydrogen and not helium as helium's boiling point is around -270'C. Keep in mind that absolute zero is -273'C. The next step up from liquid Hydrogen cooling at -250'C is liquid helium cooling and doing that right typically requires a jacket of liquid hydrogen to keep the helium effective. Either way though, they're near the realm of what's physically possible at those kind of temperatures.