Weight Watchers has an iPhone app. It's actually pretty intuitive and gives you access to favorites lists including meals and recipes. You can also view selected recipes, view your weight history and read success stories. It is very easy to use and other than the fact that is has to communicate with the server for each item tracked, it is very very convenient. I highly recommend this app!
At this time you can't use the app to track your weight, only review the history. I assume this will be available in a future release.