The term was proposed by Davidoff & Florance in 2000. Their editorial suggested that physicians should be delegating their information needs to informationists, just as they currently order CT scans from radiologists or cardiac catheterizations from cardiologists. They conceived of an information professional who was embedded in (and indeed, supported by) the clinical departments.
Supporters of the concept see it as a means for librarians to reinvigorate connections with the faculty/clinicians, as well as provide (putatively) superior service by dint of informationists' biomedical training. Critics complain that the idea is nothing new (perhaps most famously in an article entitled So what are we? Chopped liver?): librarians already provide superior service and clinical medical librarians have been working alongside physicians for years.
The large informationist programs in the U.S. are at the National Institutes of Health and at Vanderbilt University. A growing number of other institutions are hiring people in this role, though there is no universal definition or job description.
The term informationist should not be confused with informaticist, although some informationists do possess skills in bioinformatics and medical informatics. Stanford University and University of Washington are examples of institutional libraries which have hired biology PhD's (but no library degree) to provide informatics support.
Davidoff, F., & Florance, V. (2000). The informationist: A new health profession? Annals of Internal Medicine, 132(12): 996-998.