It works perfectly well under 1.5 with only a tiny bit more work than it takes to get it working in 1.6.
So... in some contexts, you can blame Apple's slow support for Java 1.6
That might be true if there was any extra download/configuration required by Mac users, but there isn't. The current MapTool code contains Rhino (the version included has been there since August in 2008). So there is nothing to download. Also there is nothing to configure. The code that exists in MapTool now would load Rhino is such a way that it works under 1.5 and 1.6 on any platform.
So my point of view that there is no hold up based on Apple is because
1) Rhino is already in MapTool
2) Rhino is already hooked up in a way to work with 1.5
3) It works exactly the same no matter what platform, no user intervention is required, no user configuration.