This is a helpful document, though a few years old. The NCES has a study about particpation in Distance Education between 1999 and 2000 available.

Here's the (dis)claimer ... the full report is in pdf on their page.

This report profiles undergraduate and graduate students' participation in distance education in 1999-2000. The report discusses student demographic characteristics associated with distance education participation as well as different types of distance education technology and students' satisfaction with their distance education courses compared to their regular courses. Results show that students with characteristics associated with greater family and work responsibilities tend to participate in distance education at higher rates than their counterparts with fewer family and work responsibilities.