Last week Greg Yardley of Pinch Media gave a very interesting talk to the NY iPhone developers meetup. He outline many trends in retention and use for both free and paid apps and sparked a lot of interesting discussion. I belive someone from Pinch will be speaking again at 360iDev so you have another chance to hear more details and ask questions. I recommend you don't miss it if you are in the area. If you can make the next talk, Greg has graciously made the slide available at