Interviewing for tenure track jobs: When will the committee contact your references? The title says it all. 

Does the committee typically contact your references (or request that reference letters be sent) a) before you phone interview b) after the phone interview but before the on-campus interview or c) after the on-campus interview? Can anything be interpreted from the timing (e.g. after the on-campus interview) of the committee's initiation of contact with your references (or their request that letters be sent)?
Can anything be interpreted from the timing (e.g. after the on-campus interview) of the committee's initiation of contact with your references (or their request that letters be sent)? 


As the comments on your question suggest, the culture on this issue are highly field/department/institution dependent. Below are some reasons why different time tables may be considered (or not considered) for requesting letters from your references. However, if letters are not requested with the application, then a request for letters at a later time means that your application has made it past one or more rounds of winnowing.

As far as contacting references, it is probably safe to assume that such contacts can occur at all stages of the hiring process at most institutions/departments depending on the candidate. If such a contact occurs, it is probably a good thing. If the hiring committee was not interested, why would they bother to contact your references?


  With the initial application.


This is an equitable way to handle letters of reference if you are not expecting a huge candidate pool or if your department wants to use letters of reference to help eliminate candidates that might not fit with mission and culture. For example, if no letters that accompany an application to a primarily undergraduate institution address the candidate's teaching style and/or efficacy, that may be a flag. A research institution may be on the lookout for individuals who too closely duplicate the niche specialty of existing faculty (ultrahigh vacuum single molecules spectroscopy).


  After the initial application, but before any phone interviews.


This is probably the way to go if you expect a large number of applications (or a large number of applications from candidates who are below the minimum qualifications). In this case, you can apply a first-pass filter to select only the candidates meeting the qualifications (e.g. a Ph.D. in Biochemistry or Molecular Biology preferably with a focus on proteomics) and remove the inappropriate candidates (M.S. in Electrical Engineering). 


  After the phone interviews, but before potential onsite interviews.


As mentioned, given the cost of onsite interviews, this is another logical time to check references. Likely, however, letters have already been requested, and at this stage the hiring committee may be contacting the references for further clarification. For example, "Candidate X is ABD with a proposed defense date of July 17, 20XX. We want candidate X to start on August 10. Will Candidate X be ready to defend in that time frame?"


  After the onsite interviews.


There seems to be no good reason to wait this long before asking for letters. Hosting onsite interviews is expensive (the institution or department usually pays for these, perhaps in contrast to industry). Again, references may be contacted to clear up questions. For example, "Candidate Y is completing a postdoc with you on synthetic fuels from biomass. However, Candidate Y's research presentation was on supramolecular tensors. What is Candidate Y's experience in this area?"