I haven't tried it, but you can add custom objects. I'm guessing that you create a plugin of the "missing" objects then import them into Interface Builder.
This is most likely because if you look closely, a couple of their objects are very specific like the "Plus Button".
AFAIK, all those exist (it's hard to see though) in the current SDK, it's just that most of them aren't direct objects. For example if you want a custom toolbar item, you drag over a generic object and then in the inspector change its type.
AFAIK, all those exist (it's hard to see though) in the current SDK, it's just that most of them aren't direct objects. For example if you want a custom toolbar item, you drag over a generic object and then in the inspector change its type.