The PlayStation Network appears to be experiencing a few hiccups right now, particularly in the UK and Europe. Reports are varied, with some users saying that they simply can't sign in, while others say that they're just running into lesser network issues. Friends lists not loading properly, the PlayStation Store not working -- all that kind of stuff.
PlayStation itself has yet to comment, but if these reports keep coming, then we're sure it'll only be a matter of time. We'll keep you updated.
Update: Functionality appears to have been restored now. Thankfully, Sony seems to mop up these outages rather quickly these days.