my immediate instinct is that the ATI drivers haven't played nice with Windows for the past couple of years, so they haven't been working too hard on the beta drivers for Vista. has anyone tried to use the generic SVGA drivers and set their resolution and refresh to match their card/monitor. this might be a stop-gag until the ATI drivers go final. I don't know since i use primarily SiS and Intel for my graphics.
Keep sending in your issues to ATI and Micro$haft... that is the only way they'll fix the problem.
Happy Beta-ing!