From Prosecuting Computer Crimes, a publication of the United States Department of Justice:
Best Practices for Victim Response
and Reporting
A. Steps
Before Confronting an Intrusion
Consider Using Banners - Real-time
monitoring of attacks is usually
lawful, if prior notice of this
monitoring is given to all users. For
this reason, organizations should
consider deploying written warnings,
or "banners," on the ports through
which an intruder is likely to access
the organization's system and on which
the organization may attempt to
monitor an intruder's communications
and traffic. If a banner is already in
place, it should be reviewed
periodically to ensure that it is
appropriate for the type of potential
monitoring that could be used in
response to a cyberattack. More
information on this topic can be found
on CCIPS' website at
http://www.cybercrime.gov.
Also, here are some sample NETWORK BANNER language as recommended by USDOJ and explanation for their functions, from Searching and Seizing Computers and Obtaining Electronic Evidence in Criminal Investigations, also by the U.S. Department of Justice:
APPENDIX A: Sample Network Banner
Language
Network banners are electronic messages that provide
Notice of legal rights to users of
computer networks. From a legal
standpoint, banners have four primary
functions. First, banners may be used
to generate consent to real-time
monitoring under Title III. Second,
banners may be used to generate
consent to the retrieval of stored
files and records pursuant to ECPA.
Third, in the case of government
networks, banners may eliminate any
Fourth Amendment "reasonable
expectation of privacy" that
government employees or other users
might otherwise retain in their use of
the government's network under
O'Connor v. Ortega, 480 U.S. 709
(1987). Fourth, in the case of a
non-government network, banners may
establish a system administrator's
"common authority" to consent to a law
enforcement search pursuant to United
States v. Matlock, 415 U.S. 164
(1974).
This is definitely a legal matter that shouldn't be so easily overlooked. More than likely, you SHOULD consult with your legal department (if you have one), or corresponding decision makers. Also, whatever is implemented in the banners, that being said for internal and external should probably not be redundant with already agreed Network Use Policies (probably don't want to constantly alert people about something they have already agreed on)