RubyCocoa had been a good idea at the beginning, but now, the MacRuby project is much more promising: 1. it's not an "expensive" bridge between original Ruby and Cocoa but a re-implementation of Ruby 1.9 on top of the Objective-C 2 runtime; 2. MacRuby objects are native Objective-C objects in disguise, no proxies; 3. the HotCocoa layers provides a much less verbose, much more Ruby-like syntax for Cocoa objects and methods.
Of course, MacRuby is now Intel only while RubyCocoa is still usable with PowerPC Macs. But the PPC days are numbered now. MacRuby is the future!