I believe that iOS 6 adds message (SMS and iMessage) notification support in the form of the Bluetooth MAP (Message Access Profile).
So unless your intended usage involves you being connected via Bluetooth then you cannot use this feature of iOS6.
I do not think that it is possible to monitor the reception of incoming message with the Apple's Official iOS SDK since you would need to utilize a private framework, so you would need to make it a jail-broken app.