As of, oh, right now, there should be a prompt on your PS3 asking you to upgrade to 4.45. Whatever you do, don't accept it. Not yet, anyway.
We've started getting reports from some readers that the update is bricking their system, locking their consoles on the boot-up "ribbon" screen, and a thread on Sony's official forums shows the issue is far from isolated.
It's obviously not locking up everyone's console, far from it, but there are enough already affected that you won't want to roll the dice.
UPDATE: Sony has pulled the update, and will announce when it's been fixed and safe for download.
Hi guys, we're aware of reports that the recent PS3 update (4.45) has caused. We have temporarily taken 4.45 offline and are investigating.— PlayStation Europe (@PlayStationEU) June 19, 2013
UPDATE 2: Sony has apologized for the firmware problems, doesn't have a fix just yet for those whose systems were bricked.
Problem after update to 4.45 [Sony, thanks everyone who sent this in. Sorry about your PS3s :( ]