How can I add a separator to the Place sidebar in nautilus?

This is not ideal but as far as I can see the only option you have short of filing a bug and chasing it through or writing and applying a patch.

• Open Nautilus
• Browse to a directory that you don't want to bookmark, for example /bin
• From the Bookmarks menu click on "Add Bookmark"
• Then Right click on the bookmark for /bin and choose "Rename..."
• Rename the bookmark to "----------"
• Close Nautilus and open it again
• you now have something that looks vaguely but not very much like a separator.

EDIT: Looks like there is already a bug filed for this. Add your own point of view here.

Hack on the code.

AFAIK there's no gconf (or other) way to alter the places that aren't bookmarks.