Hand-held radios and old analog cell phones will set off the cargo smoke detector in a CRJ 200. If it happens in flight, you cannot access the compartment to verify lack of fire, so you have to blow the bottles and land at the nearest suitable.

Generally most devices, especially modern low-powered ones, will not have a serious affect on most aircraft operations (except cell phones interfere with COM and mayne NAV radios). As was mentioned, the issue is one of certification and liability....the FAA needs proof, not just "pretty sure".

Separate from all safety concerns is an FCC-mandated requirement to not use cell phones because it confuses the ground-based cell nodes. An airborne cell phone has line-of-site with many cell towers and the system is not designed to resolve that many options when deciding which node to utilize.

Eventually I assume they will come up with standard for flight-safe devices, using a set frequency range and power output. The airframers will then design for that.