SCEE's said on the European PlayStation boards that today's PSN downtime has got precisely nothing to do with the upcoming Firmware 2.4 update.
"The PlayStation Network will be offline for 5 hours today from 3pm BST (4pm CEST) for maintenance work," said an admin. "It's pretty much all behind-the-scenes work and is, most definitely, not the 2.4 System Software update. Sorry, you need to wait a little longer for that one!"
Take a look.