I was lucky enough to write this story for Parade about detecting cancer by the sense of smell. It starts with dogs and moves on to technological nose equivalents. Check it out -- there's video.
The electronic and chemical noses researchers in this story are working on will do only a fraction of what a dog does -- they will look for a few biomarkers for a given disease.
While working on this story I talked to Jim Walker, a psychobiologist whose specialty is measuring the sense of smell. He's optimistic about the specialized devices researchers are working on, but overall, he says, "for many years the best we're going to be able to do is a dog."
"The dog is the best at chemometrics, taking a complicated chemical picture and making sense of it. That's why a dog can trail someone through the woods. Nobody thinks we have come close to a machine that can do that."
He's not just speaking anecdotally, since he's conducted careful laboratory research to learn just how small a wisp of amyl acetate a dog can smell. (The hard part of this research isn't getting dogs to show what they can do. It's precisely controlling the airborne concentrations of the chemical.) How small? One part per trillion.
That's why "the president doesn't get on the plane till the dog says it's okay."