> For $10 a month users register their license plate and state, then every month they get a printout of two random plates (at 75% size so as not to be confused with the car's real license plate) which you just stick to the side of your car doors.

This is probably illegal in most jurisdictions. For instance, it is definitely illegal in California. Standard licenses plates are 12"x6". 75% size is 9"x4.5". California Regulations (13 CCR Sec. 150.02) requires "facsimile or decorative" plates to be smaller than 9"x3" or larger than 15"x9".