Ok, but if it's true that gdm should not using tmp, what's this in the output of strace (it's an example, now I run xterm)?

lgb@vega:~$ strace -o /tmp/L xterm
[...]
access("/tmp/.gdmFV696T", R_OK)         = 0
open("/tmp/.gdmFV696T", O_RDONLY)       = 4

Since the file has "gdm" in its name (this is the file whose ownership goes to root:root after upgrade it seems), I thought it's a file of gdm. Maybe I'm wrong, as I've written before this is only a guess from me that it's a gdm bug, it can be other, of course. So I could recompose this bugreport as "cannot run X clients after major upgrades from remote ssh session", but I don't know upgrade of which package cause this exactly (well, sorry if my comment seems to be intrusive, but because my English is far from perfect I always worry about others not able to understand me correctly ...). By the way, it seems that env.var. XAUTHORITY has got value of /tmp/.gdmFV696T, exactly this file.

No disk issue, I'm sure.

lgb@vega:~$ mount
/dev/sda2 on / type ext3 (rw,noatime,errors=remount-ro)
proc on /proc type proc (rw,noexec,nosuid,nodev)
/sys on /sys type sysfs (rw,noexec,nosuid,nodev)
varrun on /var/run type tmpfs (rw,noexec,nosuid,nodev,mode=0755)
varlock on /var/lock type tmpfs (rw,noexec,nosuid,nodev,mode=1777)
udev on /dev type tmpfs (rw,mode=0755)
devshm on /dev/shm type tmpfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
lrm on /lib/modules/2.6.24-11-generic/volatile type tmpfs (rw)
securityfs on /sys/kernel/security type securityfs (rw)
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,noexec,nosuid,nodev)
gvfs-fuse-daemon on /home/lgb/.gvfs type fuse.gvfs-fuse-daemon (rw,nosuid,nodev,user=lgb)

lgb@vega:~$ env
GPG_AGENT_INFO=/tmp/seahorse-5VpTUR/S.gpg-agent:6245:1
SHELL=/bin/bash
TERM=xterm
XDG_SESSION_COOKIE=92ad7e3941d35ab5cf1cb600464d0b30-1204531474.834884-1282338699
CPPFLAGS=-mtune=pentium4 -march=pentium4
GTK_RC_FILES=/etc/gtk/gtkrc:/home/lgb/.gtkrc-1.2-gnome2
WINDOWID=59149577
USER=lgb
LD_LIBRARY_PATH=/home/lgb/.local/lib
LS_COLORS=no=00:fi=00:di=01;34:ln=01;36:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.svgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.lzma=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:
SSH_AUTH_SOCK=/tmp/keyring-5FJjf6/ssh
GNOME_KEYRING_SOCKET=/tmp/keyring-5FJjf6/socket
SESSION_MANAGER=local/vega:/tmp/.ICE-unix/6176
USERNAME=lgb
MAIL=/home/lgb/.maildir/
DESKTOP_SESSION=default
PATH=/home/lgb/.local/bin:/home/lgb/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
<email address hidden>
GDM_XSERVER_LOCATION=local
PWD=/home/lgb
LANG=en_US.UTF-8
GNOME_KEYRING_PID=6175
GDM_LANG=en_US.UTF-8
GDMSESSION=default
HOME=/home/lgb
SHLVL=1
CFLAGS=-mtune=pentium4 -march=pentium4
GNOME_DESKTOP_SESSION_ID=Default
LOGNAME=lgb
XDG_DATA_DIRS=/usr/local/share/:/usr/share/:/usr/share/gdm/
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-cpBm0IG9ur,guid=73d41efe1a2c829852672f1947cbb117
DEBFULLNAME=Gabor Lenart
<email address hidden>
WINDOWPATH=7
DISPLAY=:0.0
COLORTERM=gnome-terminal
XAUTHORITY=/tmp/.gdmFV696T
_=/usr/bin/env