The "Settings" tab of the "Advanced..." dialog should facilitate
chosing a cipher.  The list only includes the option "Default" (in
which case no --cipher option is added to the openvpn command line at
all) and all ciphers listed by "openvpn --show-ciphers".  It does not
include "none" to disable encryption, prohibiting the connection to an
unencrypted VPN.

You can work around this limitation by injecting the key "cipher" with
value "none" manually into the gconf database.

Cheers,
    Sven