Power savings requirements in wake-up circuits and RFID applications can greatly benefit from the emerging RF MEM technology. In this paper we report an RF MEM power sensor that can simultaneously offer significant power savings and co-integration with wake-up CMOS circuitry. The RF MEM sensor has no DC power consumption (in contrast to solid state diodes) and can be actuated by an RF signal as low as tenths of uW. We investigate a design and operation exploiting an RF MEM capacitive switch biased near the pull-in voltage. The device has the remarkable feature of tunable power sensitivity by the applied DC bias. We experimentally demonstrate that the RF MEM power sensor principle can be efficiently applied to a wideband range of signals. We report a study on the influence of the different parameters to better understand the performance (power savings and improved sensitivity with respect to standard capacitive sensors) and the limits of this device in terms of robustness/repeatability. An application for semi-passive RFID tags operating in the ISM bands (2.4-5.8 GHz) has been proposed, showing the feasibility of ultra-low power implementations of the wake-up circuit.