use VMware Fusion to delete bad driver
I ran into this problem two days ago when installing Boot Camp and Windows 7 x64 on my 27" iMac. After I installed Boot Camp 3.0 and rebooted, the screen blacked out due to the bad display driver, as people have reported. I fixed it by rebooting into Snow Leopard, opening up my Boot Camp partition in VMware Fusion, and deleting the bad ATI driver that had been installed (Start menu -> command prompt -> right click, run as administrator -> run "del C:\Windows\System32\drivers\ati*.sys"). Then I rebooted into Windows, it used the default display driver, and I installed the Boot Camp 3.1 update, after which all was well.