Apple obviously felt the sting of a recently settled lawsuit as they have added a warning to free apps that have in-app purchases. This will allow parents to easily identify which free apps offer in-app purchases and hopefully prevent their kids from going crazy with micro-transactions.
The warning doesn't appear to be present on the mobile App Store or on the Apple website. It is confirmed to be showing in the App Store that is contained within iTunes. We do expect it to eventually show up on the mobile version of the App Store.
An interesting idea that could spring from this new warning is the ability to filter apps that allow in-app purchases. Apple has clearly identified free apps that contain in-app purchases. Now all they need to do is implement a parental filter to prevent those apps from being used or downloaded. Apple has not said that this feature will be implemented.