Its not that PDs are being naive, especially those in competitive specialties. They know you have a back-up plan, heck...they hope you have a back-up plan. Do PDs in different specialties talk to each other about their interviewees? Unlikely.
But what most of you are forgetting is that the residents do. I used to see students all the time around the hospital interviewing - in the cafeteria, outside the ORs, etc. In their darkly pressed suits, its hard not to see them. And it would take very little, even in a large hospital, to recognize someone you've interviewed and to talk to the residents in the other program they are now interviewing for. Coming from a surgical specialty, I probably wouldn't know the Derm or FM residents, but I certainly know the surgical subspecialties, IM, EM, Rads, and Anesthesia residents and depending on my interest, I definitely might say something. Of course this depends on the randomness of seeing you suited up for your other interview, but it happens. I recall a post here on SDN about someone recognizing a student he had interviewed on another interview.
The fact is that most people who are interviewing for two specialties - one competitive, one less so - are not torn between the fields, but rather are using it as a back-up. You cannot blame PDs or call them naive to recognize this and to wonder about your commitment. Having seen my share of people claim to be interested in general surgery and then bail when that coveted Ortho or Plastics position opens up, its a bitter bill to have invested that time into someone who was planning on leaving from the day they interviewed.