Quite where it will all end up, but Apple has been awarded a US patent for embedding a heart rate monitor in a smartphone.
U.S. Patent No. 8,615,290 is entitled 'Seamlessly embedded heart rate monitor' and talks about using this functionality to "identify or authenticate" a user, or perhaps "determine the user's mood from the cardiac signals".
In this way, it doesn't seem as if Apple is looking to use the technology for health or medical features, rather for identity purposes. But it will be interesting to see whether it opens up access for thirdparty app developers to integrate with.