"full" Creates one file for all tables (default).
"tables" Creates for each table within each database a seperate file.
drush -r /path/to/drupal multi-sql-dump --bzip2 --comment=before-update-to-6.15 --target=$HOME/db_backups
The result files are bzip2 compressed and stored in 'db_backups' in your home dirctory.
Dump name schema for the command as executed above.
'example.com_before-update-to-6.15.sql.bz2', where example.com is one of your sites.
1.5.8