David,
That checkbox and the "separated by" list are still there in my copies
of LO 3.4.6 (Mac OS X, Ubuntu 11.10, Windows 7) -- IF you are using
the LibreOffice Open/Save dialog boxes. However, I note in Windows 7,
when using the Windows Open/Save dialog boxes, the checkbox is still
clickable but the "separated by" choice is as you say. We state in the
Preface to the book that we use in illustrations the LO dialog boxes,
which are the same across platforms, and that what you (the reader)
see may be different. In this case, I have added a few words about the
different label on Windows.
--Jean