Common knowledge to every overclocker is that higher clock speeds increase the heat produced by the CPU. Increased heat output will be a problem engineers may overcome with exotic cooling methods. However there's yet another deterrent which may be holding back further evolution of integrated circuits: Noise.
Kish believes that any further increase in the density of computer chips means that they will reach a physical limit -- due to thermal noise -- relatively soon. This limit is expected to come into effect around 40nm and below - and could cause problems as early as in six years time.More information @ Physics Web