On [K]Ubuntu, add the following two lines to your sources.list:
Hoary:
deb http://www.mpe.mpg.de/~ach/kubuntu/hoary ./
deb-src http://www.mpe.mpg.de/~ach/kubuntu/hoary ./
Breezy:
deb http://www.mpe.mpg.de/~ach/kubuntu/breezy ./
deb-src http://www.mpe.mpg.de/~ach/kubuntu/breezy ./
Reload the sources using apt-get update, or synaptic, and download/upgrade digikam.
(For other distros, and source downloads, see the official site)