Wide area augmentation is not new, its been under consideration since the mid 90s at least, and mainly refered, then, to on-route navigation unless local area augmentation is included such as airport located differential GPS reference station or psuedolites the latter used to provide additional ranging signals to improve dilution of precision (DOP) geometries. This is the main problem with GPS in the vertical channel. DOP is a geometric satelite orbital configuration issue as seen at the user location that constantly changes as the satelites move in their 12 hour orbits. This is one reason why there is a push to increase the number of space vehicles in orbit to mitigate occurance of DOP chimneys (peaks). Prediction of DOP chimneys is possible so that during unaceptable DOP periods aircraft can be duely forewarned caveating GPS aided navigation - not good if you're caught in the middle of a CAT II/III landing.
GPS as a sole means system down to CAT III is a delicate issue. One of the areas WAAS tries to adress is the integrity and aviability of the GPS signal for life critical appications. CAT III requires stringent availability and integrity figures which are better met by ILS systems. Getting a consensus on these figures is one poblem, DoD supreme control of GPS is another. The civil WAAS system is built around a free in-times-of-peace military system hardly a solid foundation on which to build. Things may change when dedicated civilian systems come online, designed from the outset with cooperative overlay systems in mind suitable for aviation.
Accuracy figures provided are likely 2 sigma averages under good DOP conditions.