As to why RIM would suddenly start sharing one of their most valuable exclusive features, here's what Boy Genius hears is the plan:
As far as what Android and iOS users can look forward to, we've been told RIM will offer stripped down versions of the BBM experience BlackBerry owners know and love. That way, Android and iOS users can communicate with practically anyone who has a smartphone using BBM, but they might not be able to share photos, location, or videos (when RIM crosses that bridge). Users who want the full BlackBerry Messenger experience will still need a BlackBerry smartphone to get it.
What do you think, would you use BBM on your iPhone or Droid? I'm personally not sure I need yet another way to send a two-sentence message to somebody, but we'll see.