in the latest video-sequence about geotagging the images, Jeff and Michael are talking about external devices which create the logs.
There is in my h.o. a much better way to do so. Becasue many people carry an iPhone on photo-location, there is a great tool called 'Geotag Photos pro' by tappytaps.
When started, you give the future logfole the name you want it to have, synchronize the date and time of your camera very easily ( the program shows exactly the date and time as well as the time-zone to put into your camera), and then go out shooting. When returning to the Mac / PC, simply export the file from the iPhone and directly impirt it into Lr like Jeff demonstrates. The program directly creates the file in the required GPX ( global position exchange) format.
I've checked the functionality on several trips and found it to be absolutely great and worthwhile on iPhone 4.