If this is the work of "Anonymous" members, they failed to read their own nebulous organization's press release -- Anonymous supposedly backed off from PSN attacks in early April. It's entirely possible, however, the latest hackers are members of no group other than the loose worldwide confederation of jerks.
Update: PlayStation Blog has provided another update on the outage, explaining, "we are working around the clock" to bring the PSN back online. The update adds, "Our efforts to resolve this matter involve re-building our system to further strengthen our network infrastructure. Though this task is time-consuming, we decided it was worth the time necessary to provide the system with additional security."