Well, Michel wasn't the only one who ran into this. I was able to reproduce this issue on my work machine, which forced me to either downgrade my graphics drivers and kernel or switch to the catalyst blob. In the interests of expediency, I decided to install catalyst until glamor worked again... Maybe I'll try to switch back one of these days.

For anyone interested:
I've got a Radeon 5400-series, but there were weird graphical artifacts in any kernel newer than 3.5 (I was testing 3.8 rc's when I finally switched to catalyst), although I don't remember the 3.6 series too well. Switching to glamor helped to fix this (I'm assuming that EXA/2D is to blame), but then when 1.13 showed up in xorg-edgers it broke glamor.