IN georgia:
It is not illegal for an unmarked vehicle to pull a vehicle over, however the law states that if the police vehicle is not striped according to the laws setforth in the Georgia annotated, and performs a traffic stop, it is legal but the suspect vehicle does not have to stop until another Marked unit arrives and you can not be charged for fleeing or attempting to elude. It is also legal for an "slick top" to perform traffic duties as long as their primary duty is not a "Traffic Unit", this is where people get confused about "unmarked" and "slick tops", they can't differntiate between the two.