One Yank bare-CPL holder flashed me his licence and asked me (before TO) if he could use his GPS down the back. It was a Garmin specificaly for pilot use. As these units are designed for operation in a cockpit they therefore have minimal interference. So I said yeh, but turn it off imediately if I have any problems up here. This he agreed to. We had no probs with any EI from his unit (we were 737-300 EFIS).
I dont mind authorising use of such units as long as the guy proves to me he knows what hes doing and I have a look at the toy he wants to play with. A pilots licence of CPL or above suffices in most cases.