Windows 7: ATI Radeon 4670 Display Port Problem

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?

display port SHOULD work, but you may want to see if you can lower the resolution.

Yes, I have tried playing with the Catalyst Control Center, e.g., decreasing the resolution, the refresh rate, etc., but nothing changed, the monitor remained black.

Does anybody else use ATI Radeon 4670 with external monitor via a DisplayPort cable?