The source is included in the .iso. The engine source is in darkplacesblablabla.zip. The gamecode source is in /data/zzzzzz99999blablabla.pk3 (qcsrc etc). So you allready have it :)
Source for the media (.blender files) is in the git repo on the page. The sourcecode also exists in the git repos. Extra stuff like DVD stickers exists in the mediasource git repo.
From time to time a 15GB tarball cut into pieces is uploaded which has everything all set up how it is developed.
