Recently on debian-legal someone asked whether you may freely distribute works created by a US government entity in other countries than the US. Well, I've asked a guy working on international copyright law, and unfortunately the answer was "no". Even though these works do not enjoy copyright protection in the US, they are still protected in other countries.
In international copyright law there is the so-called "principle of national treatment". This is also required by the Berne convention and similar international treaties, so any country that has signed it has to respect this principle. It means that in any state party to these treaties US nationals (and thus US government entities) enjoy the same rights as local citizen under domestic law. Since in copyright law, these rights involve the exclusive right to distribute, modify, and create derivative works, we are not allowed to do all these things we normally love to do in the free world.
So, if you are working on an open content project and think about using works of US government institutions, don't do it. It's a trap.
Copyright 2006--2011 Hendrik Weimer. This document is available under the terms of the GNU Free Documentation License. See the licensing terms for further details.