This was an idea that I had a while ago but was reminded by a posting today in Engadget (Pity I didn't patent this first)
Essentially today people use a device called a holder meter to monitor their heart rhythms. This is a portable device that you wear and it records your heart rate. The device I personally used had 3 leads and then a box to record the readings and then you transmitted the readings audibly over the phone once a day. It would be much less invasive and easier if we had this iPhone app that has external leads to monitor your heart and then allowed you to electronically submit the readings online
The issues that I faced (why I didn't pursue this) is that you would need FCC approval for the device as its a medical related machine.
Here's the link http://www.engadget.com/2010/05/06/apples-seamlessly-embedded-heart-rate-monitor-could-turn-the-ip/
As you can see from the apple post, theirs is not aimed as medical device, but I believe it has great potential in that field.