Doug Johnson is way more articulate than I will ever be so I don't think I can add much information to his warnings about online safety. However, I can add a couple of anecdotes that illustrate his points.
What you put on the internet can indeed be more harmful to you than what you read or see on the net. A few grade seven girls having a sleepover were playing around with their techno toys and posted naked movies and photos of themselves. Needless to say, word spread like wildfire and those images were viewed by other students from their school.
Two students at my high school impersonated two teachers on MySpace. They posted photos and comments and interacted with students as if they were the teachers. The two male teachers were horrified when they found out and wanted to go public to denounce this and to reassure other students that they had not really been chatting with them online. Administration and police convinced them to keep it under wraps to avoid copycat situations. One of the teachers was especially worried what his female students must be thinking of him and how they were interpreting his actions in class and "his" messages online. My own son, who has had that teacher for classes, just scoffed at the whole situation saying students would realize that it wasn't really the teacher on MySpace. I disagree. Teacher/student isn't an equal relationship in power, age, authority, maturity, etc. I think there is a huge amount of room for misunderstanding.
I think back to my first weeks in this Meet the Stars course when I was reading Star Signs by Shelley Hrdlitschka. One of the themes that jumped out of that book was a warning that people are not always who they appear to be on the net. Characters in the book misrepresented themselves to the main character and she was hurt by their betrayal. Some people argue that the net is a great equalizer. People can interact without consideration given to their physical appearance, race, handicaps, age, sex, etc. Although this can be a great equalizer there are also dangers inherent in interacting with someone who may be concealing part or all of who they really are. It cannot be stressed enough to students that they must not reveal personal contact information online.
While at an elementary school I used Blackboard, available through SaskLearning Centralischool for my students to discuss, interact and post online. I liked it because it was password protected. Many of the tools we have learned about in this class are out on the world wide web for all to see and read. I am hesitant about using some of them. For one thing, most of them (as Jane found out) require you to sign up for accounts and passwords. I'm not sure I want to require that of students. Also several of the tools had students posting photos of themselves on the net, like the assignment to illustrate vocabulary words with photos. I don't want to seem like a ludite here but I think some caution is needed. This year at school we had to have parental signatures on a media release form to even have students pictures included in the yearbook and student directory. I'd definitely ask for administrative permission before I'd put student work out on the net.