Comment 16 for bug 629622

To make the new font the desktop default either the theme could be adapted to load the font, or "sans" which is an alias for a list of preferred fonts could be updated to include ubuntu in the preferences list, I think that is in the fontconfig package and would be a change to /etc/fonts/conf.avail/45-latin.conf and possibly /etc/fonts/conf.avail/40-nonlatin.conf (although I am not familiar with how the whole fontconfig thing works). Anyhow at the moment fontconfig maps sans to DejaVuSans by default.
using sfddiff ttf-dejavu/DejaVuSans.ttf ubuntu-private-fonts/UbuntuRegular.ttf to compare the glyphs gives the attached output (it segfaulted, but not before it did the glyph comparison.)