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The new ratings system will require developers submitting new and updated applications to include a rating according to Android Market guidelines. Developers will be able to specify their own rating, but the Android team may choose to re-rate applications if users flag them as incorrectly rated.
A word to the Android developers out there: Should you ignore the request for content rating, your application will be treated as "Mature" by default once the ratings are made visible to users, according to Eric Chu of the Android Developer Ecosystem.