I'm hearing from a couple of different sources that Apple's App Store reviewers now have some way to scan submitted applications to detect the use of private APIs. I've never been an advocate of using Private APIs, and Dave and I strictly avoid them for the book examples, but I always thought it strange that Apple left to the honor system any use of private APIs that couldn't be easily discerned.
Looks like you need to step more carefully now if you have used any private framework APIs.
I know this step will annoy some developers, but in the long run, it's for the best. Private APIs add fragility to an application, and they also discourage people from submitting enhancement requests, which are how Apple gauges whether a currently private API should be made public.