Regularly scheduled maintenance on the PlayStation Network was postponed on Thursday and will begin right now. The service will be completely offline until about 5 a.m. EST.
The maintenance extends to the PlayStation Blog itself, so if you're looking for updates on the network and when it may return, follow the official PlayStation Twitter feed, which will send a notice when PSN is back online.
Once maintenance begins, everyone will be booted off PSN and will be unable to sign in. Further, PSN-enabled Web sites will be unable to serve users. Basically, any PSN site that you can sign into will be down.
PlayStation Network Scheduled Maintenance on Sunday [PlayStation Blog]