I've just tried using the Sony GPS-CS1 for my vacation recently. The Sony GPS-CS1 is about the size of a D cell battery.
The problem of trying to remember where you've taken the photos has been solved with this nifty GPS gadget from Sony.
Firstly, follow the time on the GPS receiver to your camera onboard time as you'll need the same time stamp for the GPS tagging to work successfully.
Photos of the Sony GPS-CS1.