Several of the Nikon DSLRs understand NMEA, so can be connected to a GPS. D1H, D1X, D200 and presumably most of the others. Red Hen Systems does an adapter so a Garmin unit can be mounted on top of the camera. Nikon offers the MC-35 NMEA to USB adapter cable, whatever that might be. My guess is a simple serial to miniUSB adapter. The GPS info goes into the EXIF info.