We venture beyond the ``listen-before-tal'' strategy that is common in many traditional cognitive radio access schemes by exploiting the bi-directional nature of most primary communication systems. By intelligently choosing their transmission parameters based on the observation of primary user (PU) communications, secondary users (SUs) in a cognitive network can achieve higher spectrum usage while limiting their interference to the PU. Specifically, we propose that the SUs listen to the PU's acknowledgement information to assess their interference on the primary receiver (PU-Rx), and adjust radio power accordingly to satisfy the PU's outage constraint. We investigate both centralized and distributed power control algorithms without active PU cooperation. We show that the PU feedback information inherent in many two-way primary systems can be used as important coordination signal among multiple SUs to distributively achieve a joint performance guarantee on the primary receiver's quality of service.