As I understand it, basically. Granted, I've never used BBM before.
If you try to text someone with iMessage, it will send them an iMessage instead of a text (assuming you have iMessaging capabilities). Aside from being free, the only other real difference seems to be typing notifications.
There is also upcoming/beta desktop client integration coming with Mountain Lion, fwiw.
Except I don't think BBM ever made text messages look like BBM messages. Does iMessage show you a "read" mark like bbm or what's app?
I would agree that any device only messaging system (BBM/iMessage) are probably things to stay away from if you value an easy future choice without telling everyone to stop using the old method.
If you try to text someone with iMessage, it will send them an iMessage instead of a text (assuming you have iMessaging capabilities). Aside from being free, the only other real difference seems to be typing notifications.
There is also upcoming/beta desktop client integration coming with Mountain Lion, fwiw.