On my last server to upgrade to Hardy, MySQL failed to start. Not sure why it only happened to this server, but trying to start MySQL resulted in:

/etc/init.d/mysql: ERROR: Using expire_logs_days without log_bin crashes the server. See README.Debian.gz

Sure enough, looking in /etc/mysql/my.cnf showed that log_bin was commented out, and expire_logs_days was enabled.

Uncommenting log_bin = /var/log/mysql/mysql-bin.log resolved the issue.