It's been down for several days but it went online for a sec then they reannounced that it will still need to be worked on again for another several hours. Atleast online was enabled it's just that PS store is still under maintenance.
Officially maintenance was scheduled for 1600 GMT/UTC 17th January - 0400 GMT/UTC 18th January. They gave two days' warning for this.
The maintenance overran and they posted an additional notification that it was unlikely to return before 2200 GMT/UTC 18th January. A further notice said it wouldn't return before the morning of 19th January. This was the last notification given, so one may assume that the full functionality has not yet returned.
Here's the official blog post and this is their Twitter account. One or both should be updated to reflect the end of the maintenance.
Edit: And as I post that:
PSN maintenance is now nearly complete. Service is expected to resume at around 10.00am GMT.
yeah I ain't doin' NUTHIN' til I see everyone is happy again....at this point, I'm not even signing in without a tin-foil-lined collander on my head and a ground strap attached
