student/teacher e-mail

The New York Times has an article today about student e-mails to professors, and how the tone and expectations of these e-mails differ greatly from face-to-face communication. (For any sagehens reading, it included a very short mention of a Pomona professor and how she advises students to handle e-mail communications with teachers.) As a T.A., I get surprising e-mails all the time; e-mails that, like the ones in this article, expect me to explain the day's notes to students who skipped class or which use very informal language. I'm not surprised at the latter, actually--I am, after all, a T.A. and not a professor, and try to cultivate an approachable persona in the classroom--but I do sometimes find their demands and expectations to be way too high. At least I haven't been asked for advice about school supplies . . . yet.

I wonder how long it will take for e-mail etiquette to become codified in the way business letter etiquette is, and for "rules" of sending e-mail within educational or professional situations to become common knowledge. Are there already books about e-mail etiquette? Is e-mail generally too informal of a medium to really take to such rules? Do instructors need to create a list of their own expectations for communication over e-mail? I already let students know that I won't guarantee replies after 7pm, and that I won't read complete revised drafts over e-mail, but I'm wondering if more guidelines would be useful.