Wikipedia says:
The song has been rumored to have been inspired by actress/singer Jennifer Love Hewitt; However, it was written and initially recorded long before the two even met. Mayer once said during his "As/Is" tour, that the song was about "girly parts", but this was later denied. During his "VH1 Storytellers" performance, Mayer stated that he wrote the song about his first girlfriend at age 14.
but i dunno...