Thanks, Chero.
I guess i found one cause of the problem I'm running into: It seems the upgrade (including some X stuff) had not finished yet even though xfce4 loaded again (by itself). I noticed the hdd light kept flashing when I started playing with the new environment.
Then when I tried apt-get install gdm, a message popped up asking me to do
dpkg --configure -a
... and doing so a whole bunch of processes resumed ...
So i gotta use some patience during upgrading and shouldn't be over-excited.
I'll see if I can do without xdm or gdm, as I find the system sometimes inconvenient to be forced to boot into X all the time.
Now I am thrilled to taste how Debian rocks as an *authentic* distro on Z. EABI will surely make things even better!
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[UPDATE]
My suspicion is confirmed. After waiting for the whole upgrading process to finish (which took quite a while), simply startx brings me back to xfce4, without startxfce4, xdm or gdm... No tweak is necessary. Nice.
It seems dlj0 has already fixed the X issue found in upgrading to Lenny earlier on.
Now another question pops up:
How to do mouse right-click as in pdaX? Fn+click doesn't work as assumed. Is there anywhere I can set in xfce4?