I've been running testing, but hadn't updated very many packages because I didn't want to break anything. Now that squeeze is soon to be released, I've been updating a few packages at a time. I didn't have any trouble until I went from kernel 2.6.30 to 2.6.32.
Now, parts of the screen will go black (I restarted X, and now they go gray instead). The right things will show up if I make the screen redraw (scroll/switch windows/reload web page), but sometimes it turns black (gray) again after a split second. The areas that go black follow the outlines of the gui widgets - i.e. a scrollable area will go black, or a row of tabs, or the title bar of a window, or a column on a web page.
Going by the mtime on the files /var/lib/dpkg/info/*.list, I was using the latest version of ati's graphics driver for 6 days before updating the kernel, and didn't have trouble.
$ apt-show-versions fglrx-modules-dkms linux-image-2.6.32-5-amd64
fglrx-modules-dkms/testing uptodate 1:10-6-1
linux-image-2.6.32-5-amd64/testing uptodate 2.6.32-18
According to aptitude, all packages are up to date.
I tried to take a screenshot using ksnapshot, but when I did everything redrew correctly for ksnapshot's 1-second delay, then it went back to being screwed up!
I'd like to report a bug, but I don't know what package to report it against. I find it unlikely that the kernel would affect this, since it seems to be related to drawing widgets on the screen.