Comment 17 for bug 264254

I'm the guy who helped merge the lcd filtering patch upstream. Tobias analysis is right, Cairo uses the lcdlegacy filter by default if nothing is specified in the Xrm database or in the Fontconfig configuration (which is what you have from a clean Ubuntu install).
Cairo uses the lcdlegacy filter by default in order to be compatible with what it did before. This means that people using unpatched Cairo (for instance Fedora users), won't notice a difference in behavior when Cairo is updated.

However, Debian/Ubuntu have patched Cairo for some times and there is a difference between the old Debian/Ubuntu patch and what is now upstream. That difference is about what filter is used by default (oldDebian/Ubuntu patch used lcddefault by default).

What could be done to solve this issue is to modify Fontconfig configuration to use the lcddefault filter by default. This way, Ubunu/Debian users won't see a regression. I'll file a bug on Fontconfig for this.

Another option is to modify gnome-settings-daemon and gtk+ so that they can understand this new lcd filtering parameter, and change things dynamically. This would also be the first thing to do before a UI can be developed to modify the lcd filter (as Joe_Bishop mentioned in comment #2). This is not trivial to do, so I'd go the Fontconfig way first.

For people interested, I wrote a wiki page some times ago explaining the details about all of this, and what can be done next to integrate lcd filtering dynamic changes and configuration inside Gnome: http://freedesktop.org/wiki/ScreenFontSettings