Timing and pricing details haven't been finalized yet, though it's believed that the Android app would come first and will likely be a free, stripped down version of BlackBerry Messenger. That way Android and iOS users can communicate with anyone who has a smartphone using BBM, but they might not be able to get the full BlackBerry Messenger experience unless they get an actual BlackBerry smartphone.
Sounds like a smart move on RIM's part but unless they seriously revamp their BlackBerry OS -- perhaps with some of that QNX goodness -- and bring in developers, this will probably have the exact opposite effect. For many, BBM is the main reason they are clinging on to their BlackBerry smartphones, and opening up the service to other platforms might be just the incentive they need to jump ship. If RIM indeed offers a stripped-down version of BBM on other platforms there's plenty of alternatives to choose from which is certainly easier than switching smartphones altogether.