http://www.osnews.com/story/27416/The_second_operating_syste...
"What makes it even worse, is that every baseband processor inherently trusts whatever data it receives from a base station (e.g. in a cell tower). Nothing is checked, everything is automatically trusted. Lastly, the baseband processor is usually the master processor, whereas the application processor (which runs the mobile operating system) is the slave. So, we have a complete operating system, running on an ARM processor, without any exploit mitigation (or only very little of it), which automatically trusts every instruction, piece of code, or data it receives from the base station you're connected to. What could possibly go wrong? "
http://www.thinksmallcell.com/Technology/build-your-own-open...
So anyone with a little time on their hands can be that "trusted party" for everyone in radio vicinity.