fallwith

$> launchctl unload -w ~/Library/LaunchAgents/com.mysql.mysqld.plist

$> /usr/local/Cellar/mysql/5.5.9/bin/mysqld --basedir=/usr/local/Cellar/mysql/5.5.9 --datadir=/usr/local/Cellar/mysql/5.5.9/data --plugin-dir=/usr/local/Cellar/mysql/5.5.9/lib/plugin --log-error=/usr/local/Cellar/mysql/5.5.9/data/errors.err --pid-file=/usr/local/Cellar/mysql/5.5.9/data/pidfile.pid --skip-grant-tables

$> mysql

mysql> UPDATE mysql.user SET Password=PASSWORD('root_password') WHERE User='root';
mysql> FLUSH PRIVILEGES;

$> kill `cat /usr/local/Cellar/mysql/5.5.9/data/pidfile.pid`