The keel of an F1 car is where the front suspension mounts to the chassis. The standard for many years was the V-keel and was used by all the top teams. A V-keel allows for easier and wider adjustability of suspension settings, at the expense of less efficient airflow underneath the car.
In 2001 Sauber introduced the twin-keel design, which replaced a single large keel in the center with two smaller ones on each side. This allowed air to move more freely to the floor and is generally more aeodynamically efficient, so it was favored by designers who favored aero efficiency over mechanical adaptability. Every year the design evolved and the twin keels got smaller and smaller, until eventually designers tried mounting the suspension directly to the chassis and eliminating the keels altogether. This is what we call 'zero-keel'.
In 2006 Ferrari and Renault both ran the older, more adaptable V-keel. Mclaren with Adrian Newey the aerodynamicist used a zero-keel layout. So did Toyota and Mike Gascoyne, among others. In 2007 when Rory Byrne left, Ferrari switched to a zero-keel. Renault was the only big team to stick with the old V-keel.
I haven't heard anything about whether the 2008 Renault is still a V-keel design. It seems clear that the trend is toward zero-keel because of the increased importance of aerodynamics in modern F1 design. With control tires and frozen engines, aero seems to now be the biggest area of improvement.
Come 2009 and the new aero regs, every car on the grid will most likely be zero-keel. This will probably be the final year of V-keel designs, so if Renault are sticking with it, it will be interesting to see how well they do.
EDIT: Ninja'd.