BTW, --opt is enabled by default as of MySQL 4.1 (according to the man page):
As of MySQL 4.1, --opt is enabled by default, but can be disabled with --skip-opt.
As for what it does, quoting the manual again:
· --opt
This option is shorthand; it is the same as specifying
--add-drop-table --add-locks --create-options --disable-keys
--extended-insert --lock-tables --quick --set-charset. It should
give you a fast dump operation and produce a dump file that can be
reloaded into a MySQL server quickly. As of MySQL 4.1, --opt is on
by default, but can be disabled with --skip-opt. To disable only
certain of the options enabled by --opt, use their --skip forms; for
example, --skip-add-drop-table or --skip-quick.