This would work:
Code:
/usr/bin/mysqldump --all-databases -hlocalhost -uroot -pMYSQLROOTPW | /bin/gzip -9 > /home/USERNAME/backups/XXX_alldb.`/bin/date "+%Y%m%d"`.sql.gz
If you're tables are all MyISAM and you have a shit-ton of data, checkout using mysqlhotcopy.