I am using a Precision M4400 with a Quadro FX770M on Windows 7 64bit..
I recently purchased a Dell U2311H monitor. It has a Displayport input. I decided to use a Displayport Cable to connect directly from the M4400 to the monitor. The monitor would not recognize the signal and go into powersave. No image would be seen. Windows 7, on the other hand, would recognize the monitor and I could Extend or Duplicate or run the monitor by itself. For eg if I "extended" the screen. The mouse pointer would be able to move to the external screen but the montior would be in power save and I would not see it. When plugged in VGA, this problem did not exist.
Initially I was using the latest video drivers (188.40) for Windows 7 64bit supplied by Dell (188.8.131.5240 A09 ). I only managed to solve the problem by using the latest Quadro drivers on Nvidias website.(197.16 WHQL). Now the monitor works great using a display port input. No problems at all.
For those having a Displayport issues, you can consider using the latest Nvidia drivers. (hopefully it wont cause any problems on CAD etc)
Dell: Please look into releasing new drivers for the M4400. I would much rather use drivers that have been certified by Dell AND have working Displayport functionality.