Did anybody find a solution for this? I've encountered exactly the same problem. I can have a clone desktop, but can't extend. I've tried all the suggestions given here, plus uninstalled and reinstalled the driver, but nothing.
Might be worth noting, I just did a clean re-install of windows 7. It worked fine before this so it's definitely a software problem, I just can't figure out what.
Sorry, I never found a solution to this. I ended up just leaving it the way it was. I just close the laptop and use the external monitor.