A recent news story
about the use of smart phones as heart monitors reminds me of an idea along similar lines that I recently had.
There are a variety of medical conditions, such as Alzheimers or a brain tumor, that cause a gradual decrease in mental performance, gradual enough so that the victim may not notice it. Early warning of such a condition could be very valuable.
Many people, faced with a few minutes of boredom, pull out a cell phone and play a game. It should be straightforward to add to such a game the ability to monitor some simple measure of player performance such as reaction speed that, under most circumstances, is reasonably stable over time. If the measure trends down for longer than, say, a month, a message goes to the phone's owner, suggesting that he see a doctor. Just in case.
It doesn't work for everyone. It doesn't watch for everything that might go wrong. But the cost is negligible and the potential payoff from detecting a tumor or early stage Parkinson's tomorrow instead of next year could be large.