I'm using a Contax 645 with a Kodak DCS Pro 645 back. The back has a serial cable input to accept various types of data input. One of those types is NMEA (? a GPS data standard I think?) data from a GPS unit. All you have to do is get the data to the back and the back does the rest.
I'm using an old portable Garmin aviation GPS unit from my flying days. The Garmin's output cable ends in an RS232 serial interface; I had to make my own "bridge" cable with an RS232 end and a miniplug end, to fit the digiback input socket. (Found the wiring diagram on Kodak's website, somehow.) Surprisingly, given my hamfisted soldering, the whole thing works like a charm.
The GPS apparently puts out data nearly continuously; this is automatically captured in the image files' EXIF data as you shoot: lat, lon, and altitude. Only thing I have to do is make sure the digiback is set up to receive GPS input, simply done from its setup menus.
I've not put this to any serious use, because the whole assembled apparatus is so cumbersome (I made the cable too long and I'm tripping over it.) I imagine it would be painless with newer GPS units and with backs whose makers might still be suppling ready-made GPS cables!
Hope this helps.
