Sony is undergoing massive server maintenance today. Sony said the system would be going dark for 13 hours, beginning at 9am Eastern (About 90 minutes ago). PSN - the PlayStation Store, account management, PlayStation Home, and all servers for online play - will be offline during this time.
All sites hosted on the PSN servers, like the official PlayStation Blog, will also be offline during this time.
Should there be any problems, Sony will keep gamers updated through the PlayStation Twitter account, which will remain active throughout the process. Assuming there aren't, PSN will be back online at 10pm Eastern.