I've been thinking about that on and off for years, and I think the most important factor is the teacher's apparent level of respect for the students. In the classes where the students actually worked, the common thread I noticed was that the teachers of those classes visibly expected the students to work hard and do their best. And vice versa: the teachers who were the most obsessive about controlling their classes were also the ones who got the least cooperation in this from the students. (I think the causation goes both ways here.)

So if I were teaching, say, a math class, and I wanted to use Sal Khan's videos for the lecture content, here's what I would want to do:

1. Have less class time. They're watching videos outside of class, so it all balances out, and makes the videos seem like a more legitimate part of the class.

2. Let the students know that, if they just cram before tests and don't watch the videos, they'll probably flunk. Contrariwise, if they keep up with the work, they should do just fine, and if they're having trouble, I'm happy to help.

3. Keep grading standards high enough that the preceding statement is true.

4. Have the first test be difficult but not weighted heavily, as an official kick in the pants to stir laggards from their lethargy.

I think this would go a long way toward creating the right mind-set in students, if it's matched with the corresponding respect from the teacher.