Honda has addressed the flamibility issue on their page regarding Hydogen fuel cells.

The following on the Clarity's web site addresses those concern specifically. It is likely that GM engineers used similar ideas in thier vehicle as well:

Hydrogen Safety
Sensors are located throughout the vehicle to provide a warning in the unlikely event of a hydrogen leak. Should such a leak occur, the ventilation system is activated and an automatic system closes the main cut-off valves on the hydrogen tank or supply lines as necessary. The high-voltage lines are electrically isolated. Sensors provide a warning in case of grounding. In the event of a collision, high-voltage contactors shut down the source power line. Repeated flood and fire testing have confirmed a very high level of safety and reliability.

Refueling Safety
Honda has taken safety precautions with regard to refueling safety. To prevent reverse flow from the tank, the hydrogen filler inlet has an integrated check valve. The fuel intake mechanism is also designed to prevent contamination by other gases or the connection of nozzles designed for hydrogen stored at incompatible pressure levels.

No system is perfect but there is no evidence these cars pose any greater danger to the public than any standard ICE car.

Now, you can stop being a luddite-just kidding-

TJ