From techcrunch.com:

Just in time for those new year resolutions, PushupFu is a new paid-for iPhone/iPod Touch application designed to get you fit, via press-ups. There are now a lot of fitness apps available for the iPhone. Current leader WeightBot lets you weigh yourself daily. Other like iPump let you email your progress to virtual Gym buddies. However, there are none I know of which have turned exercises into a Wii Fit-style game where you can pit yourself against others on the same service – exactly what PushupFu does.

…

Using PushupFu is pretty fun. Once the iphone or iPhone Touch is strapped to your arm, PushupFu uses the device’s accelerometer to count your pushups. It will even shout instructions like “Slower!” and bitch at you if you don’t get it right, just like a personal trainer.

Source