As many other posters have noted, learning Obj-C is about 5% of the battle. Getting to grips with the APIs is the hard part and you have to learn them whether you decide to use Obj-C, RubyMotion or MonoTouch.
So if you think that Ruby is providing you with a significant shortcut here you're in for some disappointment.
On top of this, when Apple releases new frameworks, the Obj-C users get to use them immediately. People using weird bridges are giving their competition a huge time boost by voluntarily delaying their own access to the new stuff.
I worked for a company doing Mac apps in REALbasic for several years, and this was a constant hassle. Additionally, RB chose the wrong backend for their compiler (Carbon instead of Cocoa) and there were multiple years of setbacks due to Apple hemming and hawing about whether they were going to deprecate Carbon altogether and RB porting their entire framework to Cocoa. During that process, everybody who emigrated to Objective-C got their stuff to market sooner, and with access to newer libraries to boot.
I completely understand and agree with your comment re: the importance of the APIs, but I think the big win is going to be the Ruby DSLs built on top of the tool kit to make the APIs easier to use - for RubyMotion's intended audience (existing Ruby/Rails Devs). This is just the first step.....look six to twelve months out with an active developer community and think where this project could be.
actually, i have found that without the code completion rubymotion is forcing me to learn the obj-c framework and method names a bit more. i guess I could have just turned off code completion for that.
So if you think that Ruby is providing you with a significant shortcut here you're in for some disappointment.