Damn Fick and his laws. Diffusion of air through the rubber happens due to gradients in pressure. The higher the pressure, the more driving force there is for diffusion to occur. I also believe it has something to do with the quality of the rubber/metal seal at the valve. I find a great variation in the ability to hold pressure within the same brand of tube.
In the end, retention really isn't worth worrying about. Once you buy the pump, air is free (save for the few calories spent operating the pump). It takes all of one minute to pump up before a ride.