According to the official Blackberry Blog, Blackberry Messenger (BBM) will be available to the public, from as early as September 21st at 7AM EST. The availability information for each platform is as follows:
BBM will be available as a free download for Android smartphones running Ice Cream Sandwich and Jelly Bean (Android 4.x) beginning at 7AM EDT on September 21. BBM for iPhones running iOS 6 and iOS 7 will become available for each market on the App Store schedule of 12:01 AM local time on September 22. For more information, or to download BBM for Android or iPhone, visit www.BBM.com.
BBM will continue to evolve quickly. Later this year, BBM Channels will provide a forum for active, real conversations between you and the people, brands, celebrities, artists, service providers, communities and more, that matter to you. By creating a Channel, individuals and brands can engage their friends and communities in conversations sparked by their thoughts, ideas and passions. Subscribing to a Channel will let you join conversations with people who share your interests. In addition, BBM Video calling and BBM Voice calling are planned for availability for Android and iPhone in a future version.
Source: Blackberry Blog