Energy aware system operation, and not just low power hardware, is an important requirement for wireless embedded systems. These systems, such as wireless multimedia terminals or wireless sensor nodes, combine (soft) real-time constraints on computation and communication with requirements of long battery lifetime. In this paper, we present an OS-directed dynamic power management technique for such systems that goes beyond conventional techniques to provide an adaptive power vs. fidelity trade-off. The ability of wireless systems to adapt to changing fidelity in the form of data losses and errors is used to tradeoff against energy consumption. We also exploit system workload variation to proactively manage energy resources by predicting processing requirements. The supply voltage, and clock frequency are set according to predicted computation requirements of a specific task instance, and an adaptive feedback control machanism is used to keep system fidelity (deadline misses) within specifications. We present the theoretical framework underlying our approach in the context of both a static priority-based preemptive task scheduler as well as a dynamic priority based one, and present simulation-based performance analysis that shows that our technique provides large energy savings (up to 76%) with little loss in fidelity (<4%). Further, we describe the implementation of our technique in the eCos real-time operating system (RTOS) running on a StrongARM processor to illustrate the issues involved in enhancing RTOSs for energy awareness.