Control systems used in the electrical power grid cover large geographic areas with hundreds or thousands of remote sensors and actuators. Software defined radios (SDRs) are a popular wireless alternative for replacing legacy communication devices in power grid control systems. The advantages include a low-cost, extensible communications infrastructure and the ability to reconfigure devices over-the-air, enabling the rapid implementation and upgrade of control networks. This paper focuses on the security issues related to deploying reconfigurable SDR devices as communication platforms for substations and field instruments in the power grid. The security goals are to prevent the installation and execution of unauthorized software, ensure that devices operate within the allowed frequency bands and power levels, and prevent devices from operating in a malicious manner. The main challenges are to dynamically and securely configure software components supplied by different vendors, and to validate device configurations. This paper analyzes the security goals and challenges, and formulates security requirements for a trusted SDR device configuration framework.