A startup called RidRx has created an adaptor that'll allow you to mod an old stethoscope into a fully-functioning iPhone or iPod touch-enabled version of the heartbeat detector. It comes with a phone dock/holder and an app that will record and analyse the audio you capture.
It's called the iStethoscope Pro, and can't be sold for anything other than recreational use thanks to the US regulators. However, it could prove useful - if only for kids who want to prove an irregular heartbeat to their PE teachers so they're able to get out of rugby practice on a cold November morning.
The adaptor kit costs $30, but you'll need to provide your own stethoscope - which will likely set you back the best part of $100 too, unless you know a friendly Doctor. The app itself costs 99 cents and is available now on the iPhone App Store.