According to the official PlayStation blog, the PlayStation Network will be offline today between the hours of 9 a.m. to 10 p.m. (EDT).
"I’m here today to let you know that we will be performing routine scheduled maintenance," said Matthew Harper of the PlayStation Digital Platforms Community Management team. "During much of this time, users won’t be able to access the PlayStation Store, PlayStation Home, Account Management or play online. PSN enabled websites will not be able to service users, including this one."
The Sony rep has encouraged users to keep an eye on their Twitter feed for a play-by-play.
Source: Official PlayStation Blog