Yes, cell phones have a low output, so they shouldn't be a problem if they meet FCC requirements. In the rare instance that the phone is located in a very vulnerable area for the Smart's systems, it could inject a signal into a sensitive piece of electronics. So, the answer for cell phones is that they are unlikely to cause electronic problems - more a danger for their distraction of driver's attention.
Other transmitters are a different matter. An unmodified citizen's band radio should not be a problem as long as the connectors and cable aren't routed next to sensitive electronics.
Amateur radio is authorized to operate at some very high power outputs, and that's another matter. Some vehicle manufacturers have provided technical documents with specifications for installing radio equipment in their vehicles. It doesn't sound like Smart has done this (yet).
GPS is a receiver, it doesn't transmit unless you're operating something like Qualccomm's satellite tracking and communications system, but then that's a two-way system, not the Garmin or Magellan or TomTom.
If the Smart had more metal, you might be able to argue that transmitting in the cabin on a system that has no external antenna might lead to the signal being reflected around the cabin and getting into the electronics, but as one of the ham radio operators commented, he was having trouble finding enough metal to create a ground plane for his antenna, so not much risk there either.
Happy radioing and 73.
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