Many many times. You should go do a lot of reading over at
www.xtremesystems.org and see what they do over there. There are some people here who do phase/LN2, but you won't see it mentioned very often.
Generally speaking, running a CPU at "cryogenic" temperatures is considered to be difficult, expensive, and impractical over the long term, though some will use phase-change cooling over extended periods of time with good results.