According to the wikipedia page on WAAS[0], regular GPS is accurate (95% of measurements) < 2.5m horizontal and with WAAS is is .9m horizontal.

There is also an FAA graph of GPS measurement accuracy[1] which I'm guessing is the source of various "accurate to 2.2m" claims I have seen.

[0]http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System#C...

[1]http://www.gps.gov/systems/gps/performance/accuracy/histogra...