I dont have an iphone, but this looks interesting
http://www.kickstarter.com/projects/840 ... e-and-ipod
<<The XensrCase is the most precise sports motion tracking accessory made for the iPhone 5/4/4S and iPod touch (5G/4G). Measuring 3D motion at a blistering 400 samples per second, the XensrCase is not only capable of capturing speeds, location, and altitude/elevation, but also captures precise jump heights common in high energy sports. No longer is it just about the total mileage, average speed, or estimated calories burned. Now its about stairs climbed, big air jumps, g loading, spin rates, tricks, impact on landings (or crashes) and everything else about your sessions captured with precise scientific measurement where reality trumps perception.>>