You don't need probable cause for a search, you need reasonable suspicion. A declination of a search, is not reasonable suspicion. Get that through your fucking head DH. Of course, if you give consent to a search (Which i've only had one person EVER decline a search, even when they have contraband), i'm searching your car.
Checkpoints, all you need is mere suspicion and that can be only a hunch.
The aforementioned K-9 thing about animal scents is incorrect. Also, if you have an animal inside the vehicle, they'll have you take that animal out before the search. It will not affect their ability to alert to narcotics.
And if an officer has reasonable suspicion, then the wait on a k-9 unit is a "reasonable amount of time." So with most things in life, the correct answer is, "it depends."
EDIT: to throw on some caselaw:
http://www.caselaw4cops.net/searchandseizure/k9.htm