don't think this is possible
This is a discussion on Blackberry Messenger(BBM) FOR IPHONE 4 ??? within the iPhone Apps forums, part of the iPhone Apps Discussions category; don't think this is possible...
Like I have said to myself and many others, if your that hard up about BBM, first of all why would you switch before first investigating and secondly...its easy, SWITCH BACK!
You could always get whatsapp it's free on bb download but 59p on iPhone app store. It allows me to talk to both my Sis and dad who both have blackberrys.
iPhone 4 Black
Griffin Reveal Etch Case
BB: 03.10.01 (Waiting on Unlock)
Jailbroken! GP-RC5 (UN-Teathered)