The folks on the logon team wish me to remind you that the ForceAutoLogon setting does more than just log on an account automatically. They've had to deal with large numbers of people who set the key without really understanding what it does, and then getting into trouble because what they get is not what they expected.
In addition to logging on an account automatically, the ForceAutoLogon setting also logs you back on after you log off. It is designed for machines running as kiosks or other publically-accessible scenarios where you want the kiosk account to be the only account available. Even if the user manages to fiddle with the machine and log off the kiosk user, the logon system will just log the kiosk user back on.
As a result, setting the ForceAutoLogon setting effectively locks out all users aside from the one you are forcing. If you do this to one of your machines, you'd better have some other way of administering the machine. (Typically, this is done via remote administration.)