It appears that some "irregular activity" has forced Sony to reset the passwords of PlayStation Network users in North America.
A notice was posted on the PlayStation Community Forums informing users of the reset. The formal explanation is as follows.
If you are getting an error that your PSN account password is incorrect, you may need to reset it. We routinely monitor for irregular activity, and if such activity is detected, we may sometimes reset passwords of affected SEN accounts to protect consumers and their account information.
Here's to hoping this "irregular activity" doesn't prove to be a much larger problem down the road. After all, another PSN outage certainly wouldn't help the Sony as it tries to get as many PlayStation 4s in as many homes as possible.
If you need instructions on how to reset your password, consult this guide.