If you are coming from a rails background then I would highly recommend having a look at Rhodes from Rhomobile (http://rhomobile.com/). There's lots of good things going for it and it also offers in-roads to developing on devices outside of just iPhone and Android.

If you don't find Rhodes a good fit, then have a look at PhoneGap (http://phonegap.com/) which offers similar functionality on a pure HTML + Javascript solution. I can't see you being disappointed with Rhodes though...