According to bug #513121, installing the ttf-arphic-uming package
makes QT4 fonts non-antialiased, while purging it makes them look
normal again. The last message there indicates this is because
fontconfig-config does not provide an antialias configuration by
default, and so whether a particular font gets antialiased seems to be
a fragile setting based on the installation of unrelated fonts.
Should something like 10-antialias.conf be included to make things a
bit more predictable? It appears that's what Ubuntu does.