One of my big arguments against filtering teacher and student internet access at schools is because it blocks connections to amazing resources. Students should learn how to form personal learning networks, and teachers should be utilizing them in their practice as well. Case in point:
Out of the (relative) blue, @nporter threw a link at me as a result of my recent post on filter troubles. The link turned out to be a gem of an article on Edutopia written by Suzie Boss, "Stumbling Blocks: Playing it Too Safe Will Make You Sorry."
Great article. If you're at a school that filters out blogs, wikis, YouTube, etc. read it.