Same problem here, though I didn't install beryl. I got the problem since I changed my multiple screen setup (2 screens in xorg.conf, one on my monitor, the other my TV, no Xinerama or Twinview). Strangely enough switching to a single screen layout again didn't help. Even stranger, I can set backgrounds on my second screen (TV), but not on the monitor. Bug in 3.5.6?