Many PlayStation Network users in the US today may have to reset their account password.
As posted in the official PlayStation community forum, Sony has reacted to "irregular activity" with some users' accounts. MercuryV of PlayStation Moderation and Support said, "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." According to VG247, a similar requirement took place in EU, and "only a small number of users" experienced any sort of affect from the irregular activity.
Speculation indicates that these precautionary moves are Sony's way of staying proactive in the early days of the PlayStation 4, which has already launched in the US and is set to launch in EU this Friday, but Sony hasn't said anything definitive on the matter outside of pre-emptive security.
Changing the PSN password can be done on the PlayStation Knowledge Center page, or the process can be fulfilled directly from the PS4; we at PSU assume that changing the password in this case can also be done on the PlayStation 3 or the PlayStation Vita, but we have yet to confirm or deny that claim. Through whichever means is taken, the prompt for password change will require the PSN email address as well as the listed date of birth on the account.