The Wired Campus (blog of The Chronicle of Higher Education) has a posting up that describes a new technology instructors can use to determine if distance students are actually paying attention. A YouTube video about the technology is posted below.
The sample size used in this study was only 8 subjects, but definitely seemed to give promising results. I would wager that further studies will produce similar outcomes (I could be wrong, though).
The idea behind this technology is to give instructors feedback on whether the students in an online course find the material difficult to understand. This should not be the only technology used, of course, but it might provide instructors with some insight. They could also see if students are "nodding off", which could be an indication that the student is tired or the instructor is boring.