As is the way with many theoretical attacks, this one has now been taken from the realm of theory and placed firmly at the feet of reality.
Wether you are Government, Military or business, it is now abundantly clear that it is possible for mobile communications to be intercepted both trivially and cheaply. While it is accepted that three-letter-agencies have posessed this capability for some time, it is now highly likely that this capability will now be in the hands of criminals, inndustrial spies, private investigators, journalists - in fact anyone. Mobile communications now have less security than postcards so, if you wouldn't discuss your organisation's business on a postcard, it would probably be best to avoid mobile phone use.
The bottom line is, if your adversary has a couple of hundred quid to spend and a small amount of time, it would be safer to assume that they ARE listening to your conversations and reading your texts.