I only have various incarnations of ATI/AMD video cards in my laptops and desktops. I have just installed the Steam beta on one of them, running Ubuntu 12.04 64-bit and the latest fglrx proprietary Radeon driver. It's horrible. The Steam window has a really bad flickering, which makes it unusable. I managed to get some games installed by creative guess-and-click to hit the right buttons, but then none of the games run; they give me some error, but I can't read the box because of the flickering.
So it looks that so far this is compatible with nvidia only. No luck for Radeon users.