I've played around with what seems like ever alternative framework under the sun (Titanium, Rhodes, PhoneGap, MonoTouch, Corona) and have been pretty frustrated with how clunky they usually feel. However with everything its claiming and the decent price point I'm toying with purchasing site unseen...just wish I could try it for a few hours first.
MacRuby is probably a decent place to start. While it won't mirror a true RubyMotion environment, it should be close enough to get a feel for what you're in for.
