The PlayStation Network is currently undergoing "maintenance," with Sony being relatively vague on the matter. Following the announcement, it was revealed that PSN might possibly be down for a couple of days.
As it turns out, the problem might be coming from Japan. No real confirmation yet, but PlayStation RLS reports that the issue is coming from Japan and that engineers are currently investigating and attempting to rectify it.
Hopefully it'll be fixed soon. While Sony says only some aspects of PSN isn't functioning properly, not being able to log-in pretty much seals off everyone from any and all aspects of PSN.