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If you are going to eventually use RubyMotion, why not just start with that?

I have been using it for about a week based on a recommendation from another HN'er and I have been extremely pleased. It is all new development, though. I am not especially good at Cocoa or Ruby, but have found it a very enjoyable experience.

If you are looking at starting with a hybrid approach and move towards a more native UI, you might like Calatrava - http://calatrava.github.com/

Disclaimer - Two colleagues of mine wrote Calatrava.

I'm not sure why you'd want to use PhoneGap if you're also using RubyMotion, but it seems possible.