Update 1: The good news (as pointed out by NeoGAF's "Metalmurphy") is that if your account was compromised, you should have gotten an email from PSN that says your password has been reset.
Update 2: An official community moderator on the EU PlayStation forums notes the following services are offline:
Music Unlimited via the web client
All PlayStation game title websites
Update 3: This is the purported exploit as provided to Kotaku. As PlayStation services are now offline, this exploit is no longer able to be executed:
The prodecure WAS as follows:
1) Navigate to : https://store.playstation.com/accoun...d.action?token
(this is normally, via email, https://store.playstation.com/accoun...YYYYYYYYYYYYYY
with the y's being a unique token) - do not enter the code at this point.
2) Open a new tab in firefox, and go to fr.playstation.com (other pages will work too most likely), and click Login (Connexion)
3) Click Recover password
4) Enter the email and date of birth of the target account
5) Click continue, then on the confirmation page, click "Reset using E-mail"
6) Switch back to the original tab, and enter the code, then click continue
7) You will now be asked to enter a new password for the target account