Legally, a dog sniffing is not a search under the Fourth Amendment. There could be a question whether the officers have unreasonably prolonged a detention if they make a person wait for a dog to arrive.
I know in Florida a canine can be summoned to the scene of a traffic stop to check for drugs inside a vehicle. The handler is allowed to have the dog check the "outside" of the vehicle. They start at one corner and go all the way around the vehicle. If there are drugs inside the vehicle the dog may alert to the outside of the vehicle, such as around the cracks of the doors. Yes thats right their sense of smell is that good. Once the dog has alerted to the outside of the car this gives the officer the probable cause that he needs to then open the car and let the dog search the inside.
Here's the scoop. They have to have PC (probably cause). And if you refuse to let them search, they bring in the dogs to sniff the OUTSIDE of the vehicle. If the dog gets a hit, they have their PC and thus can get a warrant to search your vehicle.