Practical Questions about Using Web 2.0 Tools in the Classroom: online etiquette

As a followup to yesterday's blog about using Twitter and blogging in the classroom and having students commenting as you learn a question came up: How do you prevent & what does a teacher do about inappropriate comments when you are teaching this way and projecting comments on wall? The person I was discussing it was wary about using this in the classroom and wanted to hear opinions of people already doing it.
My initial reaction was, to give a lesson first on "Proper online etiquette" or in Hebrew "Online Derech Eretz" (online good manners) and have consequences to actions. First of all, a teacher can delete the comment. Another would be to explain the consequences of bad "online manners" and commenting inappropriately before doing it, and a consequence might be to take away ipad or computer privileges.
I would appreciate any and all suggestions and answers to this problem.