From Apple's Developer Note, PowerBook G4, dated November, 2002, Page 41:
"Two Airport antennas are built into the computer's cover, on either side of the flat panel display."
This was a surprise to me as to the location, however, I have had quite good reception with my machine and notice the complaints on the forums have subsided. When Steve Jobs said that the new Al PowerBooks had their antennas on the cover, where they always should have been, it may have been a reference to an earlier series of Ti PowerBooks where the antennnas were located on the lower body and could be massaged through the battery compartment.