It seems like this happens every week, and hey, the PlayStation Network will be down tomorrow for yet another round of maintenance. The downtime is scheduled for 14 hours, from 8 a.m. PST to 10 p.m. PST.
The maintenance will sign out everyone logged into the PSN, and no one will be able to log in over the duration. Access will also be barred from the PlayStation Store or PlayStation Home. No one will be able to access your PSN account, either on your PS device or on your computer, and no one will be able to play games online. It's a complete online blackout.
Sony has not said what the maintenance is for, so all we can do is keep our fingers crossed that this is some sort of upgrade to the system and not a massive security breach. No one wants to relive last Spring.