I imagine if you know how the signal is encoded it would be fairly easy. I am thinking you would just need to know some specifics about the targets phone, like the channel it is on ( not sure if I am right here). It would probably be pretty easy to do for someone who is familiar with cell phone signals.
What most lay people don't seem to realize is that a cellphone is just a glorified digital radio. Nothing is private because it is broadcast over RF and anyone with half a brain can listen in to both the voice and text data.