I can't really see Apple doing this, if for no other reason than Objective-C is something they can control, and Ruby isn't. If the core Ruby developers decide they want to add more features, change the language's underpinnings, etc. Apple's left to play catch-up. If Apple wants to add features, then they basically have to fork the language and hope their changes get merged in.
Apple's shown (and been shown) how much of a benefit it is to control your own destiny; even with Ruby being open-source, it's still an outside influence they have to work with or against. Objective-C, for all intents and purposes, is their language, one they can take in any direction they want.
MacRuby is a fork of the language, albeit a friendly one. Just as Rubinius, JRuby, IronRuby, and others are all forks.
I can see Apple replacing Applescript with Ruby, but just based on the fact that MacRuby uses the ObjectiveC runtime, I doubt it's really going anywhere.
Apple's shown (and been shown) how much of a benefit it is to control your own destiny; even with Ruby being open-source, it's still an outside influence they have to work with or against. Objective-C, for all intents and purposes, is their language, one they can take in any direction they want.