Social Tagging in the Classroom

Websites such as Twitter, delicious, and wordle have made social tagging easy and accessible to the public. While these sites can be of great benefit in the professional and private world, what is their place in the classroom? Do they have one at all? Sites such as YouthTwitter offer social tagging, but with limits - are limits necessary when using social tagging, or even the internet in general, in the classroom? Are we protecting our students too much?