I know this is a sacrilegious thought, but why don't Nokia and RIM (Blackberry) just port Android to their phones and be done with it? If there are custom features on their device they wish to highlight, they can modify the OS for particular devices/develop apps.
It is going to be very, very hard for Blackberry to attract as many developers to their platform as Android has - particularly given that they haven't done a great job providing good developer tools, maintaining backward compatibility, etc.
While Nokia might be able to attract developers, they will have their work cut out for them.
It also will simplify life for app developers - let OSes to port to.
Come on, Nokia, RIM - you can do it!