When I start emacs (emacs-23 branch) with -Q and evaluate below code,
I receive unexpected result.

(with-temp-buffer
  (insert "ABC \nDEF")
  (fill-flowed nil t)
  (buffer-string))

It returns 

"ABC
DEF"

But I expect 

"ABCDEF"


I expect DELETE-SPACE option corresponds to "DelSp" parameter in
RFC3676 and works so.  Plesse see attached patch.

-- 
Kazuhiro Ito