Well, the gpl driver that comes with Knoppix works fine with a Radeon Mobility 9000 M9
so it seems to be hard to determine if it will work without trying. I've heard that ATI refuses to publish the data sheets for their latest chips though. That really sucks and they should be boycotted for it but unfortunately it is hard to find anyone who plays nice these days (Nvidia is just as bad or even worse)