As I am delivering some professional development online courses for teachers I found this an interesting article in the International Journal of Instructional Technology and Distance Learning (December, 07).
Study results strongly suggest that achieving comparable completion rates for online facilitated courses relative to classroom courses requires the development of support structures comparable to existing structures for classroom and web-based training courses. >
Well worth reading if you are delivering online courses. In the same edition is another paper "Longitudinal Comparison between Online and Face-to-Face Courses in an Adult Continuing Education Program" which also makes some good points.