AFAIK, basically you can't have anything which is higher than the top of the steering wheel.
So my double A-Pillar Gauges are illegal, because the second is over the height of the wheel and therefor in the eyes of the law, impedes my vision. A single A-Pillar gauge mounted low should be fine though.
Next time your in a taxi, see where they have their gadgets up to, from what I've heard they have them mounted legally.