Typically, when a developer uploads a new version of an application to the App Store, they have the ability to create up to 50 promo codes, which they can then provide to media outlets for reviewing the application, give away to users in a contest, etc. Aside from some reasonable restrictions on their usage (you can't sell them, they expire after 28 days and are one-time use only), promo codes provide quite a bit of flexibility to developers of paid apps who wish to freely distribute their app to select individuals without having to worry about the hassles of exchanging device information and doing special ad-hoc builds.
So where is the problem? It's in the new app rating system that was released a few weeks ago. As it stands, neither the 3.0 software nor iTunes display parental warnings when using a promo code to purchase apps with a mature (17+) rating, so Apple has made the promo code functionality unavailable for apps that fall into that category. We were informed of this condition by a developer who prefers to remain anonymous.
This obviously puts developers of these apps in a bit of a bind, as well as eliminates the potential amount of sales that could come from being able to distribute promo copies. Developers in this position may be stuck doing ad-hoc builds or going back to the questionable practice of sending iTunes gift cards to reviewers.
And if you're thinking this just applies to the massive number of adult-oriented apps that have recently poured into the App Store, you're wrong. Apple specifies that any application that may contain high levels of offensive language, violence, sexual content, or references to drugs or alcohol receive a rating of 17+. But, according to Apple, apps that feature an embedded web browser or provide access to 3rd party content also automatically require the 17+ rating, regardless of the application's content or intended audience.
Here's hoping that Apple is already working on a solution to this, as the lost potential sales caused by this not only affect the developers of the individual apps, but the overall success of the App Store as well.
Update: Several developers have reported that they are now able to request promo codes for their 17+ rated apps. We have an updated post with details here.