Making mesh sensor networks energy efficient
The problem with looking at just a single frequency for activity is that you can have other people using the same band as you and could be waking you up a lot. So in order to get around this you have to have some sort of preamble that the receiver can check to see if it matches what you are expecting. The best option I know of to do this is the CC430 which includes built-in WOR.

TI also has a document that explains how the CC430 can be used in a mesh network.