Something like this blog comment from DO, but in app format through which you could log and share progress with friends.
"10 weeks ago a friend of mine came to me asking for help sticking to the Paleo diet so we set up a system where we text each other our meals at the end of the day and then at the end of the week as a whole. We use emoji's in the notes app that comes on the iphone. An apple is a paleo meal, french fries are a non paleo meal, little swimming guy means we worked out, fish means we took fish oil and the cougar means over a half hour of AR. It's been great for both of us. We committed to do this every day until the end of the year and haven't missed a beat since. It's helped both of us increase our "average speed"
It's process driven and other than some basic parameters, the goal is to text each other through 2013."