We've been conducting unplanned experiments on the effect of in-flight electromagnetic interference for many years. Every commercial flight with more than a few dozen pax is almost certain to have one or more active mobile phones on board. Since we don't observe any ill effects, I'd judge that it's perfectly safe.
If any aviation authority thought that there was a serious risk, a requirement to scan for active mobiles (a quick and inexpensive operation these days) would have been introduced. I expect that, for a suitably large amount of $$$, the necessary work could be done to certify the safety of these devices, but it's in no-one's interest to spend the money!