Comment 1 for bug 330982

Okay, fix proposed by http://ohioloco.ubuntuforums.org/showthread.php?t=626443 worked. It was necessary to purge mysql-common (if you have a package which depends on mysql-common then just sudo dpkg --purge --force-depends mysql-common; sudo apt-get install -f . In my case, it was proftpd).
How to simulate the bug (probably): install proftpd, then install mysql-server (it shouldn't prompt for a password), then purge mysql-server and try to install it again.