I recently bought a D7000, and noticed it had a GPS input. Since I already had the GPS module (see the GPS label) I was wondering if I could connect the two. It turned out to be easier than expected: the GPS module accepts 4-6V (and the D7000 outputs 5.9V) so no regulation is necessary. The GPS output is 3.3V, and my Nikon D7000 accepts that without any problems (I've read reports of other Nikons that need a higher voltage, in which case you can use the 74HCT125 as mentioned in this schematic.
The trickiest bit is finding which cables to connect, however, there are enough resources online, like this blog for the D90, that explain it rather accurately.
I put the GPS inside a mini Altoid box, and will probably use the foot that I had for my Minolta project. To activate the GPS on the camera you need to go to the GPS menu. On the LCD display you can see the status: if it is blinking it means you connected everything correctly, but the GPS hasn't locked on yet. If it is solid, it is in use. Note that the Nikon stops listening to the stream if the data isn't being used for a while, but it keeps powering the GPS (I believe even when the camera is turned off, so you may wish to unplug if you want to save the batteries).
