My plugin-hybrid has a speaker that generates noise for exactly this reason. It's annoying to me (it spoils a nearly-silent ride), but I understand the safety reason for it.

I think that over time, this will be less of an issue, for a couple of reasons: First of all, people will be more accustomed to near-silent vehicles, and their situational awareness will adapt. Also, technology—both in the vehicle (to detect pedestrians), and for those with vision problems that otherwise need to rely on hearing—will prevent collisions.