One of the first things I talk to my new students about is online social networking. Almost all of them have posted to one of the popular sites: MySpace, Facebook, Bebo. Some of them have posted things that may not be appropriate for a public classroom. 
We talk about permanent archiving sites like Archive.org
With all the web crawlers out there, my blog gets cached by one of them within 24 hours of each of my posts. That can make it hard to “take something back.”
The Web Worker Daily has a story about developing a positive online persona. They outline a proactive approach to creating positive content that is likely to be found by a Google search. In the future it is likely that employers will check Google to see what they can find about potential employees. The article made me realize I had not purchased my own (full) name domain. Two dollars later, it’s now mine.
When I was a kid, there was a guy with the same name as my dad but he lived in the next county. He was close enough that the local paper would pick up the story when he was arrested for DUI or drug possession. Our refrigerator always had a clipping or two that was a laugh for anyone that visited. With the Internet, you should know what is out there, even if it is about someone in the next county.