Don't overlook the iPhone's built-in apps, which are standard when you buy the device:
The Clock app includes a stopwatch and timer, useful for timing workout routines, intervals between taking meds and, for diabetics, testing blood sugar after meals.
The Mobile Safari browser is useful for looking up calorie counts on the go. Just enter the name of the food and the word "calories" in the search bar. For example: chipotle
The Maps app can help you find local restaurants; use your own common sense to figure out which ones have healthy food.
TheiPod app keeps you entertained while exercising; I particularly enjoy listening to podcasts when I work out.
And you can use the Notes app to keep a health journal, writing down food, exercise, and anything else you need to keep track of.
Consumer electronics have had a huge role in making Americans unhealthy; we work all day at computers, rather than outside moving around, and then we come home and sit and watch TV or play electronic games. Using the iPhone, consumer electronics can be used to push back, and put back on the road to good health.