Using a mobile may well have an affect of nav systems, but this is unlikely to be a problem when flying VFR in a Cessna. The main point is that the PIC is aware of, and gives permission for, the use of the phone. There are, however, a couple of practical problems that I can see. Firstly, it's almost impossible to hold a conversation above the ambient noise present in a light aircraft. Secondly. it is possible that using a phone aloft will confuse the network. The phone will "see" too many base stations and can easily roam between geographically dispersed base stations in an unpredictable manner. This may lead to the network concluding that the phone has been cloned and therefore barring it.
Having said that, hasn't there been a number of cases where mobiles have been used to establish contact with ATC following radio failures? I keep the number of my local tower programed into my phone just in case.