In Mac OS X 10.3 and later and Mac OS X Server 10.3 and later, you can have your computer automatically log out of your account if you've been inactive for a set amount of time. In the client version of Mac OS X 10.3 and later, you can turn on this feature in the Security pane in System Preferences. In Mac OS X Server 10.3 and later, you can set this option in Login Preferences (click the Auto Log-Out tab).
However, if you have a document open with unsaved changes, the application will cancel the logout process. This is expected behavior. Mac OS X 10.3 and 10.4 do this to prevent you from losing your unsaved data.