If you have an ATI Radeon Xnnnn graphics card, I suggest that update to the latest (at the time of writing) version 2.6.31 of the Linux kernel and you won't get dodgy screen flicker when viewing mostly black/grey content any more.
Again, at the time of writing, Ubuntu 9.10 (Karmic Koala) comes with this kernel version off the shelf and will be properly released in about a month from now.
Commended to the house.
For details of which of the Xnnnn graphics cards have improved support, look here.
FWIW, my device is:
lspci | grep Radeon
01:05.0 VGA compatible controller: ATI Technologies Inc \
RS690M [Radeon X1200 Series]
That is all.