I am sure Apple could devise a clever way such that a CF/PCMCIA GPS system would share the internal antenna used by the AirPort card, thus eliminating the extrusion found on all such cards but have provisions for an external antenna as well.
I doubt this would work very well, as the internal antennas are 1.) Tuned for 2.4 GHz (Whereas GPS is in the 1.5 GHz range) and 2.) Already in use for the Airport card.
-- Andrew Koch - KI4CUN Website: http://www.lildobe.net
Maintain thy airspeed lest the earth shall rise up and smite thee!