The issue of hybrid and electric car silence is a complicated one. Of course, these vehicles are not really "silent," as the tires and associated systems do emit some sound, though it may be inaudible with other various traffic sounds. Some are proposing a minimum sound level for all vehicles, though nothing is likely to happen until the situation is studied.
The National Highway Traffic Safety Administration plans to begin research right away into the dangers of quiet hybrids and electric cars and has plans to introduce "technology neutral" ways to combat the problem. This continues to be an interesting discussion and one we're sure to hear plenty more about.
[Source: Automotive News - sub. req'd]