Ubuntu 12.04 login returns to login page

I've been using Ubuntu 12.04 on my EeePC901 very successfully for months and have never had any problems.
However, a day or two ago I did an extensive update which went fine.

Today when I booted, it brought me to a login page.
Nothing wrong with that you'll say except I always use autologin on every single one of the OSes that I use.

So, this was strange.
Nothing for it but to login which is not difficult.
Well, no, except when the login just brings you straight back to the login page.
Over and over and over.....
Even the Guest login didn't work.

As usual, googling turns up many possible solutions including this thread which suggests:

i) Change Display manager from LightDM to GDM

ii) Alternatively, remove ~/.Xauthority

Unfortunately, neither worked for me.

But, here's the solution that worked for me right here.

Running the command

$ ls -ld /tmp

(BTW, those are lower case L's not the number One)
showed me

drwxr-xr-x  5 root:root

After running

# chmod a+wt /tmp

the output from

ls -ld /tmp

showed

drwxrwrwt  5 root:root

and now Ubuntu booted.