To uninstall MySQL and completely remove it (including all databases) from your Mac do the following:
. Open a terminal window
. Use mysqldump to backup your databases to text files!
. Stop the database server
. sudo rm /usr/local/mysql
. sudo rm -rf /usr/local/mysql*
. sudo rm -rf /Library/StartupItems/MySQLCOM
. sudo rm -rf /Library/PreferencePanes/My*
. edit /etc/hostconfig and remove the line MYSQLCOM=-YES-
. rm -rf ~/Library/PreferencePanes/My*
. sudo rm -rf /Library/Receipts/mysql*
. sudo rm -rf /Library/Receipts/MySQL*
. sudo rm -rf /private/var/db/receipts/*mysql*