From what I've noticed, program directors talk to other PDs but not to the degree where they're talking about individual candidates. Its about more general things. PDs have a lot on their plate to worry about. When each program has at least hundreds if not thousands of candidates, they're not going to get to the point where they talk about a specific 1, and if they did, that candidate would've had to have been so over the top to the point where I wouldn't expect it.
I have though noticed that when applying to fellowship, the field is much much smaller (a handful to a few dozen tops for 1-4 spots) and almost all the fellowship directors know each other, at least professionally, even if they're on the other side of the country. However I have also noticed that they too as mentioned above are in a competition for the best candidates so they too will not divulge their "secrets".
I did though get some interesting stories from PDs during my fellowship interviews, something like "darnit, last year that program took my #1 candidate and they did it again this year", or "we took their #1 guy away from them and when I saw their PD at the AAPL convention a few months later he gave me a dirty look". However this type of thing doesn't happen on the residency level.
Honestly, the paranoia this time of year is almost reaching pharmacological levels... (Or maybe it's narcissism--"I'm so special that programs will take it personally if they found out that I liked another program more...")
I think its a natural response given what medstudents are used to, and its become a learned response. Remember, when getting into medschool, there's dozens to hundreds of applicants for every one spot. Several medschools have brutal programs. Premed applicants are told of the impossible odds and the bending over backwards they will have to do. By the time they apply to residency, the fear & "kowtowing" they are accustomed to doing is not as necessary, but they don't yet know it. Almost any candidate, with very few exceptions, that got an interview is someone the program is seriously considering. So if you made it that far, they want you, perhaps more so than you want them.