Popular imageboard website 4chan, as of yesterday, has had its access blocked for AT&T customers, at least in some cities; in some cases this appears to be for only certain parts of the website (/b/ and its cousin /r9k/), for others it appears to be the entire website.
4chan is of course a controversial website at times, but has largely been left to its own devices until now; AT&T represents the largest ISP in the United States, boasting 15.5% of all customers.
The collective readership known as Anonymous has decided to take action against the apparent censorship, though not in the way one might expect based on history; instead of going the DDoS attack route, Anonymous has decided this would be counter productive to its goal, and instead is calling for a good old fashioned boycott:
"Flood the callcenters and inboxes of AT&T," they write. "Make them confirm that img.4chan.org (make sure its img.4chan.org and not just 4chan.org) is down. Then make the honest threat of service cancellation if this censorship isn't undone.
The objective of this little operation is to basically make sure that this precedent is not set. Make it absolutely abundantly clear that this is NOT acceptable to American consumers and this WILL NOT be allowed to happen, or else face financial and political suicide."
Other methods of action are in the works or planned, including promotion of a Digg article (10,142 votes as of the time of writing), and rallying with the National Coalition Against Censorship, FCC, US Senate Subcommittee for Communications, Technology and Internet, as well as the US House of Representatives.
Interestingly, an online chat with an AT&T rep saw them flatout lying about the block, and then leaving the chat when questioned further.
According to a post on reddit, at least a few users have had access restored (possibly due to all the media attention), however it is extremely slow for some.
Update: AT&T has responded in a press release this morning, stating the block was a response to a denial-of-service attack associated with IP addresses connected to img.4chan.org (a problem the site's been having recently), which was affecting some of their customers. As such, an engineer overreacted in his response to the situation, resulting in the current status. AT&T states they have no issue with the content on 4chan; their aim was to protect their customers from malicious traffic.
4chan admin Moot gave the following statement: "We're glad to see this short-lived debacle has prompted renewed interest and debate over net neutrality and internet censorship—two very important issues that don't get nearly enough attention—so perhaps this was all just a blessing in disguise."