According to SCEA, the PS Store will be offline for most of tomorrow, from 8am PST to 1pm PST. Other PSN services, presumably such as multiplay, will be unaffected. From SCEA's site:
The PLAYSTATION®Network will be taken offline for a scheduled maintenance to our network during the following times:
Start: April 14, 2008, beginning at 8:00 a.m., Pacific Time
End: April 14, 2008, ending at 11:00 p.m., Pacific Time
During this time, the new PLAYSTATION®Store upgrade will be taking place. As a result access to the PLAYSTATION Network will be restricted, including:
* No access to the PLAYSTATION®Store
More through the link.