I'm not sure how to solve the problem, but I know it is a rather serious issue,
and could lead non-techie users to drop the whole Linux thing all-together.  The
problem is that, if a user manages to fill up all of the disk space on the
partition containing his home directory, he will not be able to log in.

Thankfully, the error message that the user receives is quite good, so, for
instance, when my little friend ran into the problem (of filling up his whole
hard drive) and called me and read the error message to me over the phone, I
knew pretty quickly what to do; I talked him through switching virtual
terminals, doing a text-based log-in, and removing some files to free up space.
 The exact message reads "GDM could not write to your authorization file.  This
could mean that you are out of disc space or that your home directory could not
be opened for writing.  In any case, it is not possible to log in.  Please
contact your administrator." (at least according to what my friend told me).

Even though the message is good, it's not quite enough for someone inexperienced
with virtual terminals and the UNIX/Linux command line.  If my friend was not
lucky enough to get hold of me so quickly, he might have just booted up his old
Windows XP partition and used that, giving up on the Linux thing.  Most people
that I know, using Linux, would not have been able to solve this one on their
own.  And someone who doesn't have a techie friend like me to call would might
just be out of luck.

So, I'm not sure how this problem should be solved, but perhaps some ideas can
be bounced around.  Maybe someone could at least start by telling me why it is
so critical for GDM to write to the user's home directory.

Some thoughts I've had, which may be real far-fetched:
 * Always keep a "place-holder file" in the user's home directory that is big
enough for GDM to "do its thing", so, when they log in, GDM can merely write
over top of this file as it needs.
 * Provide some kind of "hook" that would allow each distribution to take
further steps in helping the user fix the problem.

Other information:
Some similar bug reports:
 * http://bugzilla.gnome.org/show_bug.cgi?id=135835 - This bug was complaining
about nearly the same thing, except this guy wasn't even getting the nice error
message.  I'm taking it a step further and saying we're gonna have to do better
to solve the problem for regular users.
 * http://bugzilla.gnome.org/show_bug.cgi?id=72541 - Another similar, but quite
old report.