I recently upgraded the whole Arch Linux installation on my laptop. Everything went well, untill I rebooted and found that all fonts in GDM had suddenly turned into rectangles. The whole of gnome appeared to be afflicted, since starting gnome-font-properties from the CLI immediately revealed the same problem. And also a lot of “Pango-CRITICAL: bladiebladiebla” errors. Fortunately, I was using Fluxbox at the time, so all the non-gnome stuff was working just fine, which made the repair much easier.
After some searching on the Arch forums, I found a solution. As root execute the following command:
# pango-querymodules > /etc/pango/pango.modules
And that fixed it.