One of our readers wrote in asking whether the iPhone could possibly replace the Nike sports kit and work as a pedometer with its onboard accelerometer.
Short answer? It already does. The (jailbreak) iPhone pedometer project is open source and available on Google code. The iPhone's three onboard accelerometer sensors allow you to capture force vectors and figure out when the force changes direction as you take a step.
None of the iPhone-specific solutions that I've tried from Installer.app seem to work as well as my wiimote does on my treadmill with Wii Fit. I'd imagine that the iPhone will catch up quickly once the AppStore takes off.