It's the extreme overclockers that do the little phase change systems. They primarily cool the main processor with an evaporator block, insulated from open air. Some of the guys do both the processor and the video chip. There are a few sharp guys on there doing cascade systems that run super cold.
http://www.xtremesystems.org/forums/...Change-Cooling
A discussion about a few cascade systems running various blends, and how what gas in the blend affects the parameters of operation:
http://www.xtremesystems.org/forums/...hlight=cascade
These guys sell phase change coolers for around 2 grand:
http://www.frozencpu.com/cat/l1/g49/Phase_Change.html