I would prefer iMessage as an additional app rather than integrated in to the SMS app. The reason for this is clear as we've had many many people confused on how it first worked myself included. Also if you send a message to a user who has no coverage or data disabled but the network on it will send as a picture message which we are charged for be the network. I like how BBM and SMS are separated rather than integrated. It's a good idea but still no cause to delete Whatsapp just yet.