Managing user session with devise on iPhone APP

I have a Rails application for which I use devise to authenticate my users and this works great. I now want to write an iPhone application (not just a WebUI but a proper APP) that accesses the same data and so requires the same authentication. How should I go about doing this?

I want to login using devise and keep the session open so that queries back and forth work as they do on my website. I am very new to both rails and devise.