Obama's status as a professor
Barack Obama's status as a professor at the University of Chicago has been challenged by some blog writers who fail to distinguish the various ranks of instructors in universities. Typically, "lecturer" is the title given to entry level faculty members who are not yet appointed "with possibility of tenure." After gaining some experience and some publications, one may be offered a position that offers the hope of tenure after several years of probationary status. Instructors in those positions are typically given the title "Assistant Professor." Award of tenure changes the instructor's rank to "Associate Professor." After several more years of service and substantial scholarly contributions one may be promoted once again and the title becomes "Professor." (Sometimes it is referred to as "full professor.")
Regardless of the instructor's official rank, in many universities all instructors are addressed as "professor." In some universities, instructors are addressed as "Mister" or "Doctor," depending on whether they have earned their doctorate.
Barack Obama taught at several ranks at the University of Chicago over a twelve year period. It would always have been appropriate to refer to him as "Professor Obama." Since he has an LLD, it would also have been appropriate to refer to him as "Dr. Obama." According to the *University of Chicago website, he was offered tenure but refused.