Thanks for coming to App Craft! I'm curious how your app will be structured? If users of your app can maintain an internet connection managing user profiles is pretty easy to do with Apigee. No device based storage needed. If you want to pursue the 1 device 1 user route take a look at the HTML5 localStorage API (nice docs found here:
http://diveintohtml5.info/storage.html). You can use browser localStorage to store the uuid we create for your user object, and check for it when your app is opened. If the uuid exists in localStorage then you let them proceed, and if not ask them to register.