It hasn't been that hard for me to translate Objective-C code into Ruby. The hardest thing has been APIs that have varargs that end in nil for some reason.

Recently the RubyMotion team published this yardoc that translates all of the existing APIs for you into Ruby:

http://www.rubymotion.com/developer-center/api/index.html

But I didn't refer to that for the majority of the development since it wasn't around yet...I was knee-deep in the Apple docs for most of it.