Comment 6 for bug 1023012

Which OS? On 10.4, the standard dialog doesn't prevent you from saving a file whose name begins with a dot. You'll be warned, complete with poor wording and punctuation, after clicking Save:

| Names that begin with a dot “.” are reserved for the
| system. If you decide to go ahead and use a name
| which begins with a dot the file will be hidden.
| (Cancel) (Use “.”)

Cancel is the default (responding to Return and Escape), Use is focused (responding to Space).

It's probably still a good idea to not suggest names beginning with a dot anyway.