I think there are two big reasons that Ruby isn't on the iPhone. For one, MacRuby is still in development and isn't stable yet. For another, the Objective-C GC

I think there are two big reasons that Ruby isn't on the iPhone. For one, MacRuby is still in development and isn't stable yet. For another, the Objective-C GC isn't running on the iPhone, which is what MacRuby uses. As the project matures and the iPhone gets better hardware, I think Ruby will start to show up on it.