I wouldn't know how much the convention is enforced, but doesn't Apple prefix private API with an underscore?

Edit: this is from the Apple naming guidelines, but it doesn't clarify that they always follow that convention for private methods: Avoid the use of the underscore character as a prefix meaning private in method names (using an underscore character as a prefix for an instance variable name is allowed). Apple reserves the use of this convention. Use by third parties could result in name-space collisions; they might unwittingly override an existing private method with one of their own, with disastrous consequences. See “Private Methods” for suggestions on conventions to follow for private API.

They most definitely have private functions that do not have prefix. The iOS Three20 framework used to have a bug where you tap on a tab bar button a second time and it crashed because a cateogry method had the same name as an existing private method.

The new method could be public.