Two days ago I went to NYC, and as part of any trip down there I made a visit to the Compleat Strategist, IMHO the world's best gaming store. Anyhow, for those unfamiliar with it, they generally have the D&D/D20/Clone RPG stuff on two end caps between aisles, where most everything displayed just showing the spines of books.. I was shocked to see that they created a huge area apart from the end caps for Pathfinder stuff. Not only did PF have more shelf space than all other D&D/D20/Clone stuff combined, they were arranged nicely, where it wasn't just the spines showing like in the D&D section.
In a small store in Manhattan where every square inch of space counts, they decided to devote a massive section, relatively speaking, to PF stuff. This may be unscientific, but I tend to think of that as a fairly good indicator of how well PF is selling compared to the rest of the D&D/D20/Clone stuff.