Actually this is not really a bug: this is simply not implemented. In case of Ubuntu gdm runs xmodmap, in case of Kubuntu nothing. I guess the proper solution would be that /etc/X11/Xsession would process ~/.Xmodmap instead of gdm, like it does with all similar files (like ~/.Xresources, ~/.xsessionrc, etc.). Maybe this bug needs to be reassigned to the gdm and/or X maintainers.
Actually this is not really a bug: this is simply not implemented. In case of Ubuntu gdm runs xmodmap, in case of Kubuntu nothing. I guess the proper solution would be that /etc/X11/Xsession would process ~/.Xmodmap instead of gdm, like it does with all similar files (like ~/.Xresources, ~/.xsessionrc, etc.). Maybe this bug needs to be reassigned to the gdm and/or X maintainers.