Apple now allows developers to offer promo codes for in-app purchases
Developers like to hand out promo codes to reviewers, testers and members of the media allowing them free access to a game. Now, members of the press and reviewers will be able to install in-app items for free, allowing them to write more comprehensive reviews. Each promo code is good for 28-days from the date it is generated.
There are certain restrictions that apply to those using a promo code. For example, those employing such a code to download an app are not allowed to rate or review it. To redeem a promo code on an iPhone, iPad or iPod touch you need to open the App Store, tap on Featured and scroll to the bottom of the page. Click on Redeem, enter your credentials and then enter the code.
Promo codes can be requested in the Features section of My Apps. Only those with the Admin, App Manager, or Marketer role can request them.
source: Apple (1), (2) via MacRumors