There have already been several threads about this in the iPhone forums so I'll try not to be repetitive.
But while Beejive is great, it is no substitute for BBM, in terms of comparable functionality. BBM is included on every Blackberry and only requires a data plan. With that, you can text for free, simply by knowing the person's PIN number. With Beejive, you must know the person's IM usernames and have the app installed on your phone. All the good push apps that use multiple types of IMs also cost money.
So yes, you would still get a message while out of the app, but it is not the same, or arguably as good as having BBM. A ton of Blackberry users know about the PIN while I have my doubts that many out of the millions of iPhone users have push IM apps.