Race Buddy - How fast are you? app for iPhone and iPad
Race buddy helps you to measure acceleration from a standstill or while you are rolling. It has the ability to record video and overlay all your data (speed, temperature, G-force, the difference in altitude, etc) over your video and you can share it with your friends.
Supported external Bluetooth devices are RaceBuddy ONE (free), RaceHF Bean (PRO), Racelogic VBox Sport (PRO), Dragy (PRO), and RaceBox.cc (PRO)