When playing WoW with the latest Mesa from git, sometimes black lines and gaps
on the screen. These black spaces appear momentarily as I move around, like
intense shadows. Sometimes, the entire screen appears to go blank.

export RADEON_DEBUG=nohiz makes no difference, and no "acquiring HyperZ"
messages are printed on the console.

This is with a T60p laptop with a M66GL (RV535?) chip, running a 32 bit Linux
3.6.9 kernel.