I found this an interesting article in the Chronicle's Wired Campus. Definitely something that needs more & more intensive study, though, to prove anything or change teaching methods (IMHO).
"The conventional wisdom among educators that students’ attention tends to drift off after 15 minutes is wrong, according to a new study conducted with eye-tracking devices.
The study, conducted by David Rosengrant, an assistant professor of physics education at Kennesaw State University, found no pattern in when students become distracted. Instead, students’ focus waxes and wanes throughout a lecture and is strongly affected by factors such as where in the lecture hall the student is sitting.