MacRuby has already reimplemented most of Ruby's classes on top of equivalent Cocoa ones (String < NSString, Hash < NSDictionary, Array < NSArray). That removes a big part of bridging problem that existed with things like RubyCocoa.

Not to mention the ability to call existing APIs and the fact that C and MacRuby will both be running on LLVM—I think it's less crazy than it appears.