If you've tried to log in to the Sony PlayStation Network today, you might have been rejected. Don't worry - you're not alone.
The official Sony "AskPlayStation" Twitter page confirmed that PSN is undergoing maintenenace, so it will be difficult to access the service throughout the day. (Supposedly the problem stems from an attack by a hacker group, but Sony hasn't confirmed anything in that regard.)
Difficult, but not impossible. I tested logging into PSN via PlayStation 3 around 1:00PM ET this afternoon. I went through about five minutes of repeated error messages before I was able to successfully login. Fortunately, to play DVDs, Blu-ray discs and games (on discs), this shouldn't propose any problems for PS3 users.
However, if you want to access online games or other services like Netflix, you'll need to login into PS3 first before you can watch anything instantly.