I am sorry, but this has to be the worst idea I have ever seen. If the airbag were to deploy, that phone, mount and all, would be forced into the driver's chest at high speed. This would be a fatal accident that could have been avoided by simply NOT mounting anything on the airbag surface. Look up airbag deployment in realtime via google and you will see we are dealing with shotgun like forces, all of which would propel the cell phone directly at you at close range.
If anyone wants to follow this idea, then I welcome you to the Darwin Award's ceremony the year you encounter a frontal impact accident.