According to an article I saw, as of Oct 2003, only about 25% of the Southwest fleet had the new interiors (of course, by now, that may be a little higher). The article also said the retrofit program had been accelerated so all of the planes would have the new look by the end of 2005. It was originally scheduled to be done in 2010. I personally have not been on any Southwest planes with the new interior yet.
Hope that helps a little! Bet someone will come up with better numbers soon.