From Winkpass Creations:
Running Shoe Tracker for iPhone keeps track of the distance you put on your running shoes and to let you know when it is time to replace them. Create a record for each pair of your running shoes. Note the brand, model, and purchase date. Indicate how many miles you would like to put on each pair and/or the length of time from purchase that you'd like to replace them. Each time you run, open it, simply select a shoe and indicate distance covered.