Audrey Jaeger and Kevin Eagan of NC state have found that that when adjuncts teach introductory level courses, students are less likely to go on to take other courses in that field. The two looked at four state schools in the American southeast and found that when adjuncts taught "gatekeeper" courses, like Biology 101 or Chemistry 101, students were significantly less likely to go on to take other courses in the field.
The study was presented at the American Educational Research Association, and reported in the Chronicle here. Jaeger's web page also gives this citation for what seems to be the peer reviewed version of the research
Jaeger, A. J., & Hinz, D. (2008 - in press). The effects of part-time faculty on first-year freshman retention: A predictive model using logistic regression. Journal of College Student Retention: Research, Theory & Practice, 10(3)..
Interestingly, the effect appears only for part time adjuncts, and not graduate students or non tenure track visiting faculty. This lets us know something about what in the mistreatment of contingent faculty really begins to hurt students. Students have very little access to part time adjunct teachers, and the teachers have extremely difficult schedules and no institutional support, even something basic like an office.
My division uses adjuncts for 60% of its courses, and as a two year college, everything we teach is a intro level course. This should tell the administration something about how counter-productive their strategy is.