How much text do you want to display? You could display your organization's Acceptable Use Policy, which should define what users can and cannot do. A common logon message is "For Business Use Only. Unauthorized Use is Prohibited." Another common logon message is "WARNING: Unauthorized access to this computer system is prohibited and is subject to criminal and civil penalties." Finally, the following link provides some examples of logon messages used by the DOE: http://www.cio.anl.gov/warning.html#requirement.
For more information on this topic, visit these other SearchSecurity resources:
Best Web Links: Authentication/Access Control
News & Analysis: Authentication questions and answers
This was first published in March 2002