According to this:
http://img193.imageshack.us/img193/2523/1248672053880.png [imageshack.us], this was an ACK attack, which causes problems not only for the directly attacked host, but for other users as well.
Ordinarily, a TCP connection is set up when you send a SYN packet to a website, such as 4chan, and then 4chan responds with a ACK, and then you respond again with a SYN-ACK.
Here is how an ACK attack works. I, the attacker, will send a SYN packet to 4chan, but I am pretending to be you, or your IP address. 4chan then sends an ACK packet to you, excepting a SYN-ACK in response. However, you did not initiate the connection, so you send a RST back to 4chan (or nothing at all, depending on your firewall settings).
Then I do it again. And again. I effectively flood both you and 4chan with meaningless traffic. Your traffic problems are even worse, because if you have a firewall blocking the RST packets, then 4chan will send you 4 ACK packets (depending on configuration) for every SYN packet I send them.
In this case, AT&T and other ISPs decided that the simplest solution to ending this DOS against their users was to block packets to and from 4chan (or a specific part of 4chan).
