In GNU Emacs 24.0.50.1 (i386-mingw-nt6.1.7600)
 of 2010-09-20 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 6.1.7600
configured using `configure --with-gcc (4.4) --no-opt --cflags
-Ic:/imagesupport/include'

I've found that setting markers with C-SPC in the minibuffer for C-x
C-f and then completing some file name with TAB, the markers will be
deleted and so C-u C-SPC won't work.  This is a corner case of
completion and markers, but seems like it should be done right.  I
have proposed a patch.  The comment in the function I changed suggests
that marks should be preserved.  That doesn't seem to be the case at
all.

2010-09-30  Aaron S. Hawley  <aaron.s.hawley@gmail.com>

	* minibuffer.el (completion--replace): Try to preserve markers by
	only replacing the changed text.

--- minibuffer.el	2010-09-21 14:10:35.139030100 -0400
+++ minibuffer.el	2010-09-30 08:48:11.327387800 -0400
@@ -490,6 +490,13 @@
   ;; to keep markers "at the right place" and be robust in the face of
   ;; after-change-functions that may themselves modify the buffer.
   (goto-char beg)
-  (insert newtext)
-  (delete-region (point) (+ (point) (- end beg))))
+  (let ((pos (compare-strings (buffer-substring-no-properties beg end)
+                              0 (- end beg)
+                              newtext 0 (length newtext))))
+    (if (numberp pos)
+        (setq pos (1- (abs pos)))
+      (setq pos 0))
+    (forward-char pos)
+    (delete-region (point) end)
+    (insert (substring newtext pos))))


-- 
In general, we reserve the right to have a poor
memory--the computer, however, is supposed to
remember!  Poor computer.  -- Guy Lewis Steele Jr.