At the time of this writing, the PlayStation Network is offline, reportedly due to a distributed denial of service (DDoS) attack that occurred in the early hours of Sunday, August 24. Shacknews reports that other services and online games, such as Path of Exile, World of Warcraft and League of Legends were also attacked, but those seem to have recovered.
It's worth noting that this attack, if true, is not comparable to the 2011 PSN outage. Whereas the incident three years ago was due to compromised systems and an intrusion into Sony servers, a DDoS attack is much simpler, and basically floods the targeted network with data so that it can't make new connections to legitimate users.
Should the issue escalate or resolve, we'll keep you posted.
Update: Sony is aware of the issue and working to resolve connection problems, according to the official PlayStation Twitter account.
Update 2: Sony has posted an update to the PlayStation Blog, clarifying that "no personal information has been accessed."