The answer to this question is going to depend on how you want to deploy this app. Do you want it running completely on the phone or can it be a web app disguised as an iphone/android app?
For the native app solution I’d checkout the Rhodes Framework. It’s not RoR but you’ll see that it’s an MVC framework that feels similar. This will allow you to build native iphone/android apps using Ruby.
For the 2nd option, web app disguised as a mobile app, I’d recommend Sencha Touch. Sencha has done an amazing job mimicking the look and feel of native iphone/android apps with their Javascript library. With this solution it would be just like any other web app though it’s targeted for android and iphone via it’s UI.