the law states, "front car plate must be on the farthest point forward of the car.", hence putting it in your ground effects grill is illegal, putting it in the dash is illegal, etc...
Best way I have found so far not to get hassled is this.
like Sir Nuke said, put the lisense plate where is can be read, in dash, AND suction cupped against the glass, otherwise you can't really read it. If it is JUST in the dash, and even you can only sorta read it, a cop will kill you. If you make it very easy to see most officers wont give you anything. The BEST way tho is to get front ground effects, they have no mounting holes, AND mount the plate in the window. Not only do you not have a place to mount the plate on the front of the car, but you made it available anyways (you're nicer than corvettes). Even an evil cop will give you props.