Well there is very little information about hooking up your D3100 with GPS devices - and after much research I thought I would share the method that I went with.
First off, I opted to not use any of the "wired" methods as I think its inconvenient to do so for many reasons (I don't like taking up my hot shoe with things other than a flash, I hate holding a camera with wires hanging everywhere, etc..) In addition, most of the "wired" options are outrageously priced. And most of the "wired" units are powered by your camera's battery and thus are a nasty little power drain.
I went with the bluetooth option - A bluetooth adapter + bluetooth GPS receiver. This option will assure you MUCH less battery drain because the GPS receiver has its own power source. You'll only be powering the adapter which uses very little of your battery
This is how I did it:
I bought a bluetooth adapter that plugs into the GPS connection slot - it was called the AK-N90 and I bought it for $39.99 @ http://www.semsons.com/akblgpsadfor.html - the device is very small and it is still easy to handle the camera with it plugged in - personally, I hardly notice it. As stated above, the power drain is extremely small in that you would have to try very hard to notice the extra power drain on the battery.
Total cost for this option: $64.00 - Thus far it has worked perfectly all the time. Got a trip to Italy coming up, I'll let everyone know how it holds up over there. Either way, as it stands now, I would highly recommend this option. This option should also work well on the D90.