Maxim Integrated Products has an RMS power detector designed to operate from 800MHz to 2.0GHz.
It accepts an RF signal at the input, and outputs the same voltage regardless of the peak average of the input signal.

The output voltage and input power is linear in dB. The device has a -25dBm to +3dBm detection range, and every dB change in input power gives 40mV typical change in output voltage.

With 0.1dB typical RMS accuracy from WCDMA modulation change and ±0.5dB detection error due to temperature, the device is designed to improve closed-loop performance of the transmit path in multiband, multimodesystems

It can also be used for SAR limiting.

For more information, see http://www.maxim-ic.com