I've heard from 3-4 different places with these types of letters and/or emails. It seems that others that I know on the interview trail are hearing the same thing from the same places; therefore, I'm not so sure that it really means much. However, I do think that your response to these probably does matter.
My (totally uneducated) opinion is that the PD would not be sending these if they did not plan to at least rank you. For programs in which I am only mildly interested (mid-list), I have written back to the PD, telling them how much I enjoyed my interview visit, but since I still have interviews remaining, I "owe it to myself to complete all of my interviews before making a final decision." I have heard from 2 places that I do plan to rank very highly. For those programs, I have written very positive notes back to the PD and then contacted the program coordinator for extra information (information on housing, realtors, etc.). That type of information is very valuable to me, as my wife and I plan to buy a house, but it probably also conveys sincere interest.
Although programs say that they do not require "second looks," I feel that you can show them that you are very interested by gathering more information. You'd be surprised at how much information makes it back to the PD's....I called a realtor in a city at the recommendation of someone from the Department of Surgery there. Four days later, I got an email from the PD saying that he had heard I was interested in local real estate and offering some additional advice. Apparently, the realtor called the person to thank them for the referral, and the info got passed up the line!
Summary is: be careful with your response; they will be listening closely.
