Comment 11 for bug 972279

I've just run into this problem (again, usually I just changed the font in Kate). The link in comment #6 seems to be the key, and the package responsible is fonts-droid, which causes the default monospace font to change from DejaVu Sans Mono to Droid Sans Mono which indeed doesn't support bold. I tried uninstalling the package but it's a dependency of texlive-full. The fix would appear to be to lower the priority of the droid fonts but I don't know what else that depends on.

I'm running 12.04, but at this point it's rather a horrible hybrid so my install doesn't really count for bug reporting purposes; I'm running both the kubuntu-ppa and texlive-backports repos.