In an official warning email distributed to people who use their Uplay service, Ubisoft announced that their servers had been attacked and that user information, including user names and encrypted passwords, in the account database had been compromised.
We recently found that one of our Web sites was exploited to gain unauthorized access to some of our online systems. We instantly took steps to close off this access, investigate the incident and begin restoring the integrity of any compromised systems.
During this process, we learned that data had been illegally accessed from our account database, including user names, email addresses and encrypted passwords. Please note that no personal payment information is stored with Ubisoft, meaning your debit/credit card information was safe from this intrusion.
Ubisoft has recommended that anyone with a Uplay account change their password, and also recommended that you change the password on any other sites on which you use the same password.
For a while, Ubisoft's site had been displaying an "under maintenance" message, but as of this writing appears to be back functioning normally, which would indicate that they got things sorted out.