As a developer, I honestly could not care less about a health-spying app that comes bundled (and presumably will be as undeletable as Apple's stock, weather and passbook apps) with the iPhone. I consider such built-in apps as little more than bloatware, so I truly hope Cook & Co. don't really plan to pretend this will be of interest to the general population of developers.
As one of 371 million diabetics, I am actually quite interested in this. I am also very physically active (partly to control diabetes) and would love a great system to track movement, etc., especially if it were paired with the iWatch to gather info like my heart rate. So, maybe not that many developers will be interested, but I think there is no shortage of customers for those developers who are interested to sell to.