-- Last Updated: Oct-01-12 2:58 PM EST --
Maybe listening to people saying that "its all based on water molecule size - vapor vs liquid" is not the best of ideas. The size of water molecule in a block of ice, a jug of water and a cloud of fog are exactly the same - hard to believe, but amazingly true. The key words here should be "wetting", "hydrophobic", "surface tension" and "diffusion".
The essence of any "membrane" out there is that it is basically a spider-web of hydrophobic "threads" that are packed close enough to be impermeable to liquid water due to the effects of surface tension (the fibers are not wettable). However, the spaces between the fibers allow water vapor (individual water molecules - for simplicity sake) to make their way though the membrane. The speed of this transport is dependent on the properties of the membrane and also on the temperature and humidity gradients across the membrane - if the outer layer of the "breathable" fabric becomes wet (saturated/covered by layer of water) the rate of diffusion will drop by order of magnitude, if not more. This is why all "breathable" fabrics have some sort of water-repellent coating on the outer surface - not to add to the water-resistance, but to keep the outer surface "dry" so to allow vapor to escape.
If you read somewhere that "our membrane" allows (liquid) sweat through, rest assured, it's hogwash - because if the membrane is wettable and permeable to liquid, it's wettable in both directions - ergo, it is not water resistant.