Cloud computing-based data centers, which hold a large amount of customer data, are vulnerable to physical attacks and insider threats. Current protection and defense mechanisms for security of data held in data centers are either completely physical (sensors, barriers, etc.) or completely cyber (firewalls, encryption, etc.). In this paper we propose a novel cyber-physical security defense for cloud computing-based data centers against physical attacks. In our system, physical sensors detect an impending physical/human attack which triggers cyber defenses to protect or mitigate the attack. The key to the cyber defenses is that in cloud computing data centers the data is loosely coupled with the underlying physical hardware, and can be moved/migrated to other physical hardware in the presence of an attack. In this paper we propose a model for coupling such cyber defenses with physical attack-detection sensors. We further describe a preliminary architecture for building such a system with today's cloud computing infrastructure.