Permeable concrete that allows water to pass through was on display at the 2009 Renewable Energy Roundup. Chino's Concrete of Austin put a slab of their concrete in a tank and poured water over it. A good portion of the water made its way through the concrete and into the lower portion of the tank.
The concrete has no re-bar and looks like a conglomeration of small balls of concrete.
When installing a permeable hardscape according to Brad Lancaster it needs to be installed so that it receives rainfall but not runoff, since runoff will contain matter that will eventually clog it up. So if it used for a sidewalk for instance it should be higher than the surrounding lawn.