Right now, the FAA's goal is to have a total of 800 explosive-sniffing dogs at the 80 busiest airports. The cost to train one of these dogs and its handler is $15,000. The TSA also employs dogs and their handlers.
http://abcnews.go.com/WNT/story?id=130545&page=2MSP, my home-town airport has 6 of these dogs, which do various jobs, mostly sniffing baggage. They can only work limited hours. In one case, a terminal was evacuated and delays were caused by a false alarm from one of the six dogs.
http://minnesota.publicradio.org/display/web/2010/01/06/last-bag-msp/In Philadelphia, three of these dogs failed their recertification tests and had to be retrained.
http://content.usatoday.com/communities/pawprintpost/post/2010/01/bomb-sniffing-dogs-need-to-get-up-to-speed-to-protect-travel-hubs/1All TSA bomb sniffing dogs are trained at a single facility at Lackland AFB. The number of dogs that can be trained at that facility is limited.
http://abcnews.go.com/WNT/story?id=130545&page=1Bomb sniffing dogs in Iraq were found to be poorly-trained and ineffective by the State Department's inspector general. These dogs are trained and supplied by private contractors.
http://www.salon.com/news/feature/2010/10/08/bomb_sniffing_dogsRoughly 1.5 million passengers pass through security screening each day on domestic flights in the USA. How many of these dogs would be required to sniff all of those passengers? Keep in mind that the dogs can only work a limited number of hours, so multiple shifts would be required. They also sniff out false alarms. What happens then? Is the screening area shut down? Who would train all these dogs? The current training center at Lackland AFB has a limited capacity, and private contractors, as shown above, are not necessarily producing well-trained animals.
Who would breed all of these dogs? Only some breeds are particularly good at this job. Will factory-style breeding centers be set up to produce the large number of dogs that would be needed? Each dog must also be trained with a specific handler. Where will these handlers be found, and where will this training take place?
It seems to me that such dogs are not a realistic solution for discovering explosives that might be carried by passengers. The sheer number of passengers, along, would appear to make such a thing impractical in the extreme. Add to that the limited working hours of these dogs and the false alarms possible, and it doesn't appear that dogs are the future of security screening.