And, the CPM rates are huge, and people actually do click these ads (based on my revenue numbers)
A reasonably high-end display advertiser will pay a relatively high CPM to have their brand associated with an object like an iPhone with a high perceived value, even if no one ever clicks, and especially if theirs is the only ad on the screen at a time.
With the insanely fine-grained targeting information that Apple has about you and your device ---such as your name, age, country and home billing zipcode, how much you use your iPhone, your purchase history with Apple, whatever other information you've ever given Apple like income and occupation, your current location (in principle), what apps you like, what music you like, etc.---they can probably pick the right ad for each user well enough get really, really good average click through rates. You'd be amazed at how well you can crank up the average click likelihood even with way coarser targeting information.
Based on my experience and observation of others it seems to occur fairly regularly.
(Also, I know my 4yo clicks on in-game ads by mistake all the time on my iPod, and I know I'm not the only parent who has discovered the miracle of iPhone/iPod gaming for kids.)