Just as expected, as the Android market is seeing some refinements to prepare for Gingerbread, the application rating system has gone into place, effective today. Bringing in a total of 4 ratings, all applications will have to submit a rating before they are allowed into the Android Market.
The ratings are All, Pre-Teen, Teen, and Mature and come with some requirements. Any app that will use the devices current location should be rated Pre-Teen+, and if it allows them to publish their location it should be rated at least Teen. If an application refers to alcohol, tobacco, or drugs will have to be rated Teen or higher but if the app focuses on the consumption of alcohol, drugs, or tobacco then it will carry a Mature rating.
The changes for existing apps will have to take place over the coming weeks, and if an application fails to get a rating by the developer within this time frame it will receive a Mature rating until it has been rated. Users will also be able to flag applications that they believe to have an inappropriate rating and Google has full rights to change the rating. If a developer continues to falsely rate applications, Google reserves the right to remove the applications in question from the Android Market.
You can check out further guidelines from Google's Android Market support page.