Anon was involved in a separate spate of attacks against Sony and Sony officials are complaining that they where so preoccupied guarding themselves from it, that it allowed these guys to launch a major assault with Sony not noticing until the last minute. Remember, they didn't just attack the PSN, they attacked SOE too which is a network for MMORPG games (that's another 22 million users).
Anon is denying it but Sony is claiming they found a file on their that said "Anonymous", probably just an attempt to frame them since it would be stupid to leave that behind and then deny it.
Sony took the network down out of their own will yes, this wasn't a DoS attack. They are going to investigate the damage and beef up security. I also heard that the hackers will target the Sony PS website in the coming days and post our details online.