AT&T has started blocking access to img.4chan.org, which means access to the 4chan imageboards /b/ and /r9k/ are unavailable to most (not all) AT&T customers. An article on the wiki Encyclopedia Dramatica has details:
-Most AT&T DSL (largest ISP in the US, 15.5% of US Internet users) customers ARE currently not able to access img.4chan.org (/b/ & /r9k/)
-There are a few AT&T DSL users that are NOT affected. Florida, and Ohio are where we’ve gotten those reports. (Parts of Georgia are still alright as well.) AT&T is a megacorp made from the smaller pieces it was broken down into in the 80s- different equipment and such.
-AT&T Mobility (Cell Phones) and AT&T Uverse customers ARE NOT AFFECTED.(Lie, Uverse is blocking it also)
-It IS NOT a DNS issue. Affected people have tried OpenDNS with no success.
-It IS very visible on a traceroute. It drops within the AT&T network.
-It DOES NOT affect AT&Ts Tier 1 backbone (major bandwidth backbone in US).
-It DOES NOT affect other servers on 4chan.
-People HAVE called customer service (allegedly) and confirmed a block, but agents have denied further info.
Near as I can tell, multiple customer service agents have confirmed that this is intentional, and not a bug or an accident. The ED article seems to be updated regularly. The story has also been covered on Reddit, with a very active discussion.
The apparent reason for this was to alleviate bandwidth costs on the part of AT&T due to a DDoS attack against 4chan. As far as I know, ISPs have the right to throttle connections, but not to arbitrarily block a connection like this. It appears that AT&T is breaking the law.
Of all the sites AT&T could have chosen to filter, 4chan is probably the one that’ll put up the biggest fight. Part of me is looking forward to watching what happens next. Don’t get me wrong, though; censorship is serious business. Something needs to be done.
EDIT (11:50pm, Jul 26): AT&T has begun to allow access to 4chan again.