Grab the GPS signal, convert it to 5V level and send it to the Nikon Camera's 10 Pin plug. http://www.k-i-s.net/article.php?article=20 With that, whenever a picture is taken, the GPS position is part of the EXIF data.
Let the microcontroller release the Camera Shutter. This is a functionality of above 10pin plug and would allow taking pictures in intervals, e.g. every 20 seconds. Or a a button is implemented on the remote XBee so you can operate the shutter from remote.
Another otpion would be to use that tripod for Gigapixel images. The you point the camera into one direction, press start and then the camera takes one picture, moves the head by a few degrees, takes another picture, etc and at the end all pictured are merged into one image with extreme high resolution. http://www.tawbaware.com/maxlyons/gigapixel.htm
Another option would be to remove the GPS tracking function and rather remote control the tripod from the computer. The hardware for this is simple, the droids board has an optional USB connector instead of the XBee module. Or we use an USB-to-XBee adapter available from that company as well. You would send some serial commands to the board, the software parses those commands and moves the camera head accordingly. No big challange, just the parser code has to be modified. And then via Nikons Camera Control software you could view the life image on the computer. There seems to be some open libraries (http://www.gphoto.org/) as well to build an integrated solution. Or you simply use a webcam.
Apart from these enhancements there is one particular issue. The microcontroller does not know the servo's initial position. So all it can do is setting the position to central at the beginning at that results in one very quick movement. As the servos are very strong, that does not do the entire construction very good, not to mention the force on the camera. The servos do support programming and with that you can define speed or read the current servo position. And that would be all that is needed. Implementing the protocol would take a while so it will not be possible for me. Right now, I turn on the tripod, let it move to the initial position and then put the camera onto it.
Check with a scope the servo speed implementation works flawlessly. It is not perfect yet as you can see in the video.
See what needs to be done to take the GPS signal and send it to the camera.