I've read that it is ineffective to use a heat-pipe cooler on a vertically mounted motherboard, if the heat-pipe legs are oriented in such a way that water condenses and pools inside them (due to gravity) and never returns to the heatsink base to absorb more heat and evaporate again.
However, some manufacturers claim it is perfectly fine to position their heat sinks in any orientation that fits the motherboard. For example, in response to the question, "My Thermalright heat sink has heating pipes. Does it matter if I position they[sic] up or down?" Thermalright says, "Our heating pipe system is designed to work both ways. Having the heating pipes facing up or down depends on how they fit your motherboard best." (See Thermalright FAQ # 12
According to the manufacturer, the following AXP-100 cooler should work just as well rotated 90 degrees counter-clockwise, as it does oriented as shown:
Does this make sense? Are these manufacturers doing something different inside their heat-pipes, or are they simply relying on the thermal conductivity of the metal, rather that the liquid inside, to facilitate cooling?