The ability to limit a login to one user at a time is important to me too. I am surprised that Moodle allows multiple users to login simultaneously with one login credential. Moodle is so security conscious, I expected the security to extend to limiting simultaneous use of login credentials.
I took the solution idea posted on 11/6/05 and expanded on the code to limit access to one user logged in at a time using the same credentials. When user 2 logs in, user 1 is logged out. User one can then log back in and change his password to block user 2.
The logic also limits the login to 2 hours per day so you can't login one time and allow multiple users to use Moodle. If you log out, the system remembers the time remaining for that day so you can log back in and use the remaining time. I added a "Session Time" block so the user will know how much time he has left for that day.
This works OK in Moodle 1.9.9 but I am not sure it will work on Moodle 2.0 without some editing. If anyone would like to see my code hack, send a request to firstname.lastname@example.org.