The PlayStation Network is having some serious problems right now, and it's being caused by an internal clock bug that's only present in models released before the PS3 Slim.
In light of this, and because they're weary of the damage that could be done, Sony are urging consumers not to use their systems. A shortened version of the statement reads "if you have a model other than the new slim PS3, we advise that you do not use your PS3 system, as doing so may result in errors in some functionality, such as recording obtained Trophies, and not being able to restore certain data."
There's no concrete ETA on a fix for this, but Sony hope it will be done within 24 hours. While it's bad news for PS3 gamers, their competitors must be absolutely loving this news.
Source: PlayStation Blog