The Bluetooth specs (v4.0) state that the time between sending 2 packets is 150µs. So if one device starts listening after 150µs and the other one starts sending after 150µs then the only time one could inject a packet would be when one of the devices is also transmitting. This would cause an interference which would result either in "losing" the packet or receiving a faulty packet. Thus for the attacker an injection would never be successful and lead to a DoS in the "best case".
So is successful injection of packets possible or is ubertooth advertising an "impossible feature"?
P.S. Let me know if you know a more appropriate forum I can ask this question.