Christine,
I could also see this as a problem in a traditional class. I always have some students that have very clearly NOT prepared for class with the assigned readings, etc. The CMS systems have tracking features that you can at least see if the students are accessing the content and at what days and times. You still can't guarantee that they read the materials for proper engaged the content for learning. The tracking logs at least give you something to take to the students to encourage them to complete the course materials (i.e. "...I see you have not accessed… >>>