From the Launchpad website:
The domain name of Cairo-Dock (cairo-dock.org) is temporally down ! (2010-01-06)
The name cairo-dock.org is not directly accessible at the moment. This affects the forum, the wiki, the repository, and the themes. (not our ppa's, bzr and launchpad bug tracker).
But you can also find our website with its forum and wiki there :
http://cairo-dock.vef.fr/
It will be back soon, sorry for that...
The Cairo-Dock team.
EDIT: the come back of this domain name is not planned until 23, 24 or 25 January 2010. Please wait
EDIT 2: if you want download another theme (general, gauges, clock, etc.), you can simply use this command :
cairo-dock -S themes.cairo-dock.vef.fr