How to compile DigiKam 3.1 on Ubuntu 12.04 was this blog's most visited post by far, with the sad Tribute to Nord about our Labrador dog, who is not with us anymore, coming second by a close margin in front of digiKam installation and configuration - Part 1. All in all it looks like there is quite some interest about digiKam on Ubuntu or maybe other instructions available online are a bit outdated. Anyway, since 13-May-2013 digikam 3.2 is available and although you can guess how to compile it from comments to this post, it deserves step by step instructions.
update: I successfully tested steps below on Ubuntu versions 12.04, 12.10 and 13.04