Following is the only thing I found on the web regarding the NPS
policy on letterboxing.

----To: All National Park Service Areas and Offices

From: Division of Ranger Activities, Washington Office

Day/Date: Wednesday, March 21, 2001

Geocaching There is a new web-based activity called geocaching
that has affected several National Park Service areas. The Ranger
Activities Division asked Olympic NP SA Mike Butler to investigate.
Here's his report: Geocaching is an activity in which participants
hide a cache and take a position at the location using a GPS
receiver. The position is then published on the group's web site
with an invitation to search for the "treasure." Caches often
contain a notebook or log book and something the finder may take.
The finder is asked to put another item in the cache for others to
discover and will often report the find on the web site. Several
caches have been found in National Park Service areas. The webmaster
for the site (www.geocaching.com) has been contacted. He was very
surprised that geocaching is illegal in NPS areas, and understood
NPS concerns about the damage geocaching has and can cause to
historic, archeological and natural sites. He agreed to work with
the Service to discourage further geocaching activities in parks.
Two related activities were also discovered. Letterboxing
(www.letterboxing.org) is a phenomenon similar to geocaching in that
a player takes directions from a web site and uses those directions
to find a hidden object. In letterboxing, the directions come in the
form of a riddle and the hidden object is a stamp which the finder
can use to stamp a piece of paper to prove that he has visited the
site. The web site showed the location of at least two letterboxes
in parks. The parks have been notified, but the Service has not yet
contacted the webmaster or game managers. The Degree Confluence
Project (www.confluence.org) is another web-based activity where
people try to visit various latitude and longitude integer degree
intersections and report their findings on the web site. In this
case, however, no objects are placed in the ground, and there are no
apparent regulatory violations in areas where cross-country travel
is allowed or where the confluence is not on a protected site. There
has been no attempt to contact the project organizers. [Mike Butler,
SA, OLYM]------

Note that Geocaching and I assume by association letterboxing are
described as illegal. This is from 2001 but says that the webmaster
had not been contacted at that time. Perhaps some group needs to be
formed to lobby the NPS on our behalf. This is from the "Division of
Ranger Activities" and I would assume they merely respond to
official policy interpreting in the what ever way leads to the least
change or resistance. I don't believe this ever made it above low
level policy wonks, I think we need to ask them as a group to
provide us with an official statement from the Department of the
Interior (Which oversees the NPS and the Director who is Fran
Mainella her address is; Interior Bldng, room 3104, 1849 C St NW,
Washington DC 20240) regarding the activity then we have something
to use to begin lobbying the NPS to change it's policies. I know a
government official intimately involved with the NPS who could get a
signed lifer from Ms Mainella outlining their stance on
Letterboxing, however we have no organizational structure and
therefore no representatives that speak for the group as a whole,
NOR AM I PROPOSING THAT ONE BE FORMED, I would hate to see this turn
into anything other than an informal association of like minded
individuals, but perhaps the request should come from the webmaster
of LBNA, I'd be happy to assist.