While most of us develop our apps using Objective C and Xcode, there's always utility in investigating other tools. RubyMotion stands out among other alternative toolkits for developing iOS (and OS X) apps because it generates native code and support the use of existing Objective C libraries.
I wrote a tutorial for Objective C developers who want to pick up RubyMotion. It doesn't focus on Ruby idioms, but more as a quick introduction to the differences in tooling and language.
From the intro:
RubyMotion is a great alternative to writing iOS (and OS X) apps with Objective C. Many RubyMotion users seem to come from Ruby backgrounds and are new to iOS development. On the other hand, some are Objective C developers with little or no Ruby experience who wants to pick up RubyMotion. This tutorial is for the latter group and gives examples for iOS. The same principles and tools apply for OS X development.