iPhone promo codes blocked for 17+ apps
According to TUAW, the problem stems from the fact that iTunes fails to display any content warnings when using a promo code to download a copy of the application for free.
For the most part, these promo codes are used for promotion (as the name suggests) and sending out review code to iPhone publications. But they've also found employment in competitions and various promotional giveaways, which Apple is presumably concerned about younger users obtaining.
It is quite easy to see Apple's point of view on this subject, but the real downside is that any application featuring an embedded web browser, or that provides access to any kind of third-party content is automatically given a 17+ rating.
Considering the 3.0 software put so much emphasis on social aspects and connectivity, this could put a real dampener on the use of promo codes in general.