sudo /etc/init.d/slim stop
Replace slim with gdm or lightdm or whatever your login manager is. At this point all the text in your console might be jumbled. That seems to be another bug. Just type the following commands blind:
sudo Xorg -configure
sudo cp /root/xorg.conf.new /etc/X11/xorg.conf
Next, edit /etc/X11/xorg.conf in a text editor by adding the following line under the "Device" section where you see the "radeon" driver:
Option "AccelMethod" "XAA"
You also want to tab in the Option and make sure it lines up with the section. Then save and exit, type startx or restart the computer, and you should have your fonts back.
At first I tried to fix this with the xorg option "NoAccel" "True", but that turned off all acceleration and made my desktop unacceptably slow. But changing the "AccelMethod" from EXA (default) to XAA, it preserves 2D desktop acceleration and fixes the font issue. There's more information in these threads here and here.
Just thought I'd leave this here in case anybody googles, "WHERE THE HELL DID MY FREAKING FONTS GO LINUX WHEEZY HELP."
UPDATE: Peter S. reports he can get fonts back with EXA acceleration using the following xorg.conf edits:
Option "AccelMethod" "EXA"
Option "RenderAcceleration" "false"
And you may also need:
Option "MigrationHeuristic" "greedy"
He also says he can now use Compiz without it "lagging like crazy" on his G3 :)
UPDATE II: Here's a bug report, Bug #702480 xserver-xorg-video-radeon: GTK fonts are invisible on PowerPC