I finally decided to try the 9.12 hotfix release on my 64-bit Ubuntu 9.10 installation with an HD4870. Building the deb-files went just fine, and so did installing them. But when I start X my two monitors (connected via DVI) goes to sleep. "Hmmmm. Funny." I thought, so I tried the real 9.12 release; the same thing.
Going back to 9.11 solved the problem.
Has anyone else seen this problem? I tried searching ati.cchtml.com but it hasn't been updated with new versions of Catalyst since 9.7.