The description of the "default case" option in the Red Hat Enterprise Linux Samba documentation could be misleading

"default case = upper/lower
    controls what the default case is for new filenames (ie. files
    that don't currently exist in the filesystem). Default
    lower. IMPORTANT NOTE: This option will be used to modify the case
    of all incoming client filenames, not just new filenames if the
    options case sensitive = yes, preserve case = No, short preserve
    case = No are set. This change is needed as part of the
    optimisations for directories containing large numbers of
    files.

By default, Samba 3.0 has the same semantics as a Windows NT server, in that it is case insensitive but case preserving. As a special case for directories with large numbers of files, if the case options are set as follows, "case sensitive = yes", "case preserve = no", "short preserve case = no" then the "default case" option will be applied and will modify all filenames sent from the client when accessing this share."  The documentation could be read to imply that for directories containing large numbers of files the combination of the following settings is optimal:

case sensitive = yes
preserve case = No
short preserve case = No
default case = lower

I have read and re-read those section from the man page and find that the structure of the sentences and lack of punctuation leaves them open to a number of interpretations.