<loads of licensing stuff not pertaining to the question asked>
As part of the Tremulous media, the ATCS map is released under the CREATIVE COMMONS ATTRIBUTION-SHAREALIKE 2.5. The spirit of this licence is relatively similar to the GPL, but for media. You can modify the map and release the new version, but it will have to be under the same licence, and credit need to be given. Look at the license explanation on the
CREATIVE COMMONS website, it's written with simple words. Of course, even if it's not required by the license, it's always more polite to inform the author of the original work

Be careful, this licence cover explicitly the media released by DarkLegion for the official Tremulous release. The maps made by third party people may be licensed under other term. If I remember well (a post from Timbo), any map who want to be part of the official next release of Tremulous need to be covered by this same licence.
And just for being up to date, the actual SVN repository contain a modified version of the COPYING file I've posted above. The Tremulous team seem to have contacted ydnar, and he has re-licensed his shaderlab textures under the same CC license.
http://svn.icculus.org/tremulous/trunk/COPYING?rev=775I think it's pertaining to the question asked by Imperium.