PlayStation Network will be down for maintenance tomorrow from 12.30pm EDT (Wednesday, 2.30am AEST) to about 7pm EDT (Wednesday, 9am AEST). This is probably related to last week's console-bricking patch, as Sony generally tries to avoid performing maintenance on the same day the PlayStation Store updates.
So, PlayStation Store will be down, in addition to PlayStation Home and Account management features. Online multiplayer will be available, but only if log into your PSN account sometime either today or tomorrow before maintenance begins, Sony says.
For those concerned about access to PlayStation Plus games or other purchases, a community manager said the Store update will be released tonight, to "allow you to hop into the store tonight ahead of tomorrow’s store update post, and pick up your purchases prior to the Tuesday downtime. Since this is an early update, be sure to take advantage of any current sales now, while you can."
PSN Maintenance Scheduled for Tuesday, June 25th [PlayStation Blog]