Years ago wanting to write Mac software and having loads of experience with Java WebObjects I tried the java bridge but decided to bite the bullet and learn Objective-C (fortunately since I would have hated having my software deprecated with the bridge). Later I fooled around with RubyCocoa. I learnt Ruby (found it interesting indeed), but found out the hard way that the bridge was far from mature or stable and at the end I ended porting the code back to Objective-C.
Since years have passed, I'm wondering if it is worthwhile investing some time with MacRuby, or even learning Python to use PyObjC. As much as I like Objective-C, I recall being way more productive with the Ruby bridge when it didn't crash. I just would hate investing time to end up with crashy software again.
I would say MacRuby is the way to go if you want to try one of the bridges. It's being developed by Laurent Sansonetti, who's a Senior Software Engineer at Apple working on Ruby.
It's quite functional now, and integrates nicely with the native frameworks. Worth a look, particularly if you already have Ruby experience.