[ Thanks to finid
for this link. ]
“The guest session from the login screen enables anybody with
physical access to the computer to login, provided the screen is
not locked, and use the computer. Any data generated in the course
of using the system under a guest session is not persistent. There
is nothing inherently wrong with having a guest session from the
login screen by default, but many users seem to want it disabled, a
process that is not difficult, but does require editing a simple
but poorly-documented configuration file.“If you are not currently running Ubuntu Desktop 11.10, this is
what the login screen looks like. The guest session is an
auto-login account. No password is required to use it.”