The New York Times has an interesting article called "To: Professor@University.edu Subject: Why It's All About Me." It talks about how informal and even inappropriate students' emails to their professor can be. By inappropriate they are referring to asking questions about school supplies (to get a binder or to get a notebook, that is the question), asking for lecture notes when they've missed class (that is the student's responsibility to get that from classmates - the prof may not have readily available class notes), and so on.
As an instructor, I have noticed this informality to some degree, not in the sense that my students are requesting things of me that they should not be (no one has asked for lecture notes or anything), but rather the lack of capitalization and punctuation that probably stems from IM use. It does not really bother me, but I have noticed it.
Anyway, I think the article is a good read, particularly for students. It gives them some sense of the boundaries for email communication with a professor. Of course, this does vary among professors - some of them can be rather informal, and prefer to be that way.