What about the GM quadrasteer and the Honda 4ws car? The rear steer axle has no mechanical linkage and it is all electricly controlled. I think there are a couple other cars with electric steering controls for the front axle too.
Here are the street legal requirements for steering in Washington:
http://apps.leg.wa.gov/WAC/default.aspx?cite=204-10-034
http://apps.leg.wa.gov/RCW/default.aspx?cite=46.37.375
There are a few. I have seen them in the shop but I can't remember what ones for sure. My tech was saying a Volvo but can't remember what model. BMW is coming out with a "drive-by-wire" system. From what I read it'll be more responsive than conventional steering. We'll see..