Agreed. I would be willing to bet that 4chan is unblocked within 24 hours. Someone at AT&T did something stupid.

Indeed. San Francisco is reporting they're unblocked already. Also, it appears that it was not someone doing something stupid... the most official-looking explanation I've found is here: http://www.merit.edu/mail.archives/nanog/msg19609.html

Nutshell: the affected subdomains were getting DDoS'd, and the colo housing the relevant 4chan servers couldn't take it, so AT&T did the colo a favor and started dropping traffic upstream.

EDIT: Aaaand now everything is down.