I think the answer is simple. With the amount of people on facebook and twitter or on their computer in general, why wouldn't schools do this?
It is the easiest way to connect to students. Every person I know in university has a laptop. That means they are on the internet or on their computer for at least a couple hours a day. If their homework is posted on the internet, sent in emails or however a teacher decides to do it, they are more likely to remember to do it rather than a teacher mentioning it in a 2 hour lecture.
It saves the teacher the trouble of students "forgetting" and it allows students easier access to their work! The advantages are unlimited, along with the resources the internet offers for students.
I've watched a couple lectures in the Robert H. Smith School Of Business at the U of Maryland's itunesU courses, and they are all very interesting. All the ideas explained are something that I already had a general idea about but the way they explain it and elaborate on simple concepts really opens it up for me and raises a lot more questions than I would think there is about these ideas.