US military application of existing GPS satellites can already achieve fixes down to 5cm. Centimeter-accurate positioning will never be allowed for civilian use, no matter how many satellite constellations you put up there.
I don't know about that. Is there anything malicious that a person could do with centimeter positioning that they can't already do with meter positioning?
Hell, I doubt that that there has ever been an instance where someone has managed to commit a crime because their GPS was meter-accurate, as opposed to for example 10-meter accurate or 100-meter accurate.
If I'm not mistaken the accuracy restrictions are mainly there to prevent foreign forces from developing munitions such as these: http://en.wikipedia.org/wiki/M..., but those restrictions will be extremely difficult to keep up when Russia, China, EU and others (India, Brazil, etc) have fully operational satellite constellations. You will pretty much need to get everyone to not compete on accuracy while there will be massive demand from the market for things like more accurate autonomous vehicles.