HighFlowRod's has got it just about right in terms of the function of a Phase Change Material. PCMs do qualify as thermal interface materials and have been used as TIM for years on stock HSFs. Most have not been terribly effective in terms of thermal performance, but that has been due to the poor material characteristics and non-optimized, wide "melt-and-freeze" temperature ranges of the older PCMs. Times have changed.
The Nanotherm PCM+ is optimized for junction temperatures above 30'C and is an excellent choice for a TIM in standard, non-refrigerated air and water-cooled applications. Since it will remain a solid and not "phase change" at temps much below 30'C, it is not recommended for low temp useage. Nanotherm Silver XTC, on the otherhand, has a tremendous temperature range - both high and low - and works quite well for sub-zero apps - or even in your oven for that matter.
BTW, allow me to dispel one myth that came up in the review. The PCM+ does not turn into a semi-solid with heat. The aqueous carrier simply evaporates quickly at room temp, turning the Nanotherm PCM+ from a liquid to a semi-solid wax-like coating. The liquid nature of the PCM+ is a design feature to make most easy to apply in a very thin layer coating, which is all you need.
Here's how it works: Heat from the processor is stored in the material and builds up until the material "melts" (although it doesn't run). Once it "melts," it releases the heat and then "freezes" to a solid again. This cycle happens very quickly (milliseconds) and repeats itself over and over again, effectively maintaining a relatively constant temperature in the thermal junction. Add some thermally conductive particles to the mix and you have a dual function material that also conducts heat via direct thermal conductivity in addition to the "phase change" cooling effect.
The Nanotherm PCM+ really is pretty cool stuff