I have an ATI Mobility Radeon HD 4670 (on my Dell Studio XPS 1640 notebook) and I just tried to use my external monitor (Dell P2210) through a DisplayPort cable, but it did not work in Windows 7 (x64). It did display the BIOS and the Windows startup screen, but after the startup the (external) screen went black. My guess was that it could be a driver
issue, thus I just tried to roll back the driver, but it did not solve the problem. I currently use Catalyst 10.3 beta (driver 8.710.3.0). Do you have any idea what could be the problem?