Is it the permeability of the rubber, the valve, or both? I'm only riding my bike 100-150 km per week and I have to top up the tires every Saturday. I'm running 700C x 23's @ 120-130 psi, and in a week it'll lose 10-15 psi.
It's interesting, there has been a wide adoption of exotic materials in the bike world - carbon fiber, titanium, lots of Alu and steel alloys, now carbon nanotubes, etc.
You'd think that with all these advances we'd have a way to do something as mundane as keep air in a tube. But maybe it's just me.
Do slimed tires hold their pressure better? On my hybrid with bigger, lower-pressure tires the tubes are slimed, and it does hold pressure better, but that could be down to the lower pressure, too. In any case, I'd rather pump up the tires every week than add the weight to my skinny-tire bike. I'm just curious.