Hope you weren't planning on buying anything from the PlayStation Store today. "Routine scheduled maintenance" will make many features of the PSN unavailable for the entire day, until 12AM PST.
Unlike the last major outage, "most online play will be unaffected." Sony's Patrick Seybold says that "users who are already signed into their account on PlayStation Network before the start of the maintenance should be able to stay signed in to their online session." However, "users who attempt to sign-in to their PlayStation Network account after the start of the maintenance may be presented with the site maintenance notification page if they haven’t signed in recently."
In addition to the PlayStation Store, the following services will also be unavailable all day: PlayStation Network Account Management, PlayStation Network Account Registration, PlayStation Home, Music Unlimited, and Video Unlimited. Your Trophies will remain secure during this time. "You can continue to collect in-game Trophies and these will be updated on your profile once PlayStation Network is back up and running," the PlayStation.Blog notes.