AT&T, which services 15.5% of U.S. internet users, has blocked access to the popular imageboard 4chan, or more specifically, img.4chan.org.
The company has released a statement that they had demanded 4chan make changes to the site and these demands were not met. Moot, the founder of 4chan, denies being contacted by AT&T.
Opposition is clearly building, as the Twitter hashtag #ATTCensorship is seeing a huge surge in popularity at this hour. Google News, Digg, Reddit, and several other sites have picked this story up, and the internet culture-related wiki Encyclopedia Dramatica has been aggregating updates from around the web.
UPDATE: Please see Author's comments below for updated information.