Sony announced that it will take the PlayStation Network offline for a few hours for routine maintenance, starting tomorrow at 10:15 AM Pacific and running through 4:15 PM Pacific, approximately six hours.
During this time, a few of the key features will be down. You won't be able to access the PlayStation Store, Home and Account Management options. That said, the rest of the features should be accessible, including online play. A post on the official forums suggests that signing in before the maintenance should keep your services going through the six-hour stretch. You can read more information on that here.
Sony didn’t give specific reasons for the maintenance, but we suspect it involves improvements to the service overall.