User-Agent:       Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.5) Gecko/2008121300 SUSE/3.0.5-1.1 Firefox/3.0.5

Assume you add a statement like

keycode 115 = Multi_key

to /etc/X11/Xmodmap, which as far as I know is the correct way to make such a setting globally. You will find that indeed in the Username field of the kdm login panel one can compose umlaut characters with the help of the corresponding key. But this setting gets lost in the process of login. It happens when
/etc/X11/xinit/xinitrc.common (which is called by /etc/X11/xdm/Xsession) on line 105 calls xkbcomp. My cheap workaround consists of calling xmodmap again:

xdpyinfo | grep -q "X.Org version: 6.9.0" || \
  setxkbmap -print | xkbcomp - $DISPLAY

/usr/bin/xmodmap /etc/X11/Xmodmap

This is probably not the correct solution. xkbcomp should respect global xmodmap settings.

regards

Rolf

Reproducible: Always

Steps to Reproduce:
1. Add an entry to /etc/X11/Xmodmap
2. logout
2. Test the effect of the entry in the Username field of kdm
3. login
Actual Results:  
The setting from /etc/X11/Xmodmap wasn't in effect any more.

Expected Results:  
The setting from /etc/X11/Xmodmap should still be in effect.

Probably the fastest way to see everything, is to modify

/etc/X11/xinit/xinitrc.common in the following way:

#start of code

/usr/bin/xmodmap -pke > /tmp/A1

xdpyinfo | grep -q "X.Org version: 6.9.0" || \     # original line
  setxkbmap -print | xkbcomp - $DISPLAY            # original line

/usr/bin/xmodmap -pke > /tmp/A2

#end of code

add this line to /etc/X11/Xmodmap:

keycode 115 = Multi_key

logout
login
do a
diff /tmp/A?