Tests are performed when inflight connectivity is added to a plane -- and it has resulted in finding interference issues, such as WiFi blanking avionics screens (http://www.flightglobal.com/news/articles/wi-fi-interference...).

For cellphones, there have been many reported incidents of interference when pilots had them on (search Google). I'd assume testing on cell phones would officially commence with installs of pico cells on US airliners.