From Patrick Giudicelli:
GPS Speedometer use the iPhone 3G integrated GPS to calculate the speed. Works only with the iPhone 3G and only outdoor. The "location" option in the iPhone settings must be activated. Note: The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.