The Writer Guide (http://wiki.documentfoundation.org/cgi_img_auth.php/e/e5/0203WG3-WorkingWithText.pdf), Chapter 3 - Working with Text, page 11, mentions the key combination for non-breaking spaces is Control+spacebar. I believe the key combination has been changed to Control+shift+spacebar. The documentation should be updated to reflect this change.
reproduced in LibO 3.5.0 beta 1
site of documentation: http://wiki.documentfoundation.org/Documentation/Publications
Very old issue and no longer valid.
The referenced pdf no longer exists.
The currently available wiki chapters for 3.6.x, 4.0.x, 4.2.x describe the correct control+shift+space key combination.