Today I got my Rivals.app rejected from AppStore because it used private API calls. Fortunately nowadays Apple lists the calls in the reject message to make it easier to remove them. I was unfortunate to be using:

• addTextFieldWithValue:label:
• textFieldAtIndex:

For anybody wondering about their app prior to submitting to AppStore here is a collection of popular private APIs:
http://www.drobnik.com/touch/2009/11/forbidden-fruit-apple-apis/