Sony tweeted the news via its PlayStation Twitter account, linking to a post on the PlayStation Blog. The post notes that online play and other features of the network are still up and running, and claims an update will be provided "soon." We'll be keeping an eye out for the update, and let you know when then will be now.
Update: Users are reporting that codes can be redeemed, just not necessarily through the PlayStation Store. From the XMB menu, go to Account Management -> Redeem Code.
Update 2: The PSN is armed, but not yet fully operational. A post on the EU PlayStation Blog notes several updates, the most recent of which reads: "We have been working on the PSN service roll-out throughout the weekend and are continually increasing the availability of the voucher redemption functionality. The next phase of the roll out is to introduce the What's New Activity Feed and Live Item in the Content Information Screen, which we will be able to update you on tomorrow. Thank you for your continued patience as we manage PSN to restore full service."