Originally Posted by Xorg.0.log
(WW) NVIDIA(GPU-0): Unable to read EDID for display device DFP-3
(--) NVIDIA(0): Connected display device(s) on Quadro NVS 160M at PCI:1:0:0:
(--) NVIDIA(0): HP LP2475w (CRT-0)
(--) NVIDIA(0): LPL (DFP-0)
(--) NVIDIA(0): DFP-3
(--) NVIDIA(0): DFP-3: 330.0 MHz maximum pixel clock
(--) NVIDIA(0): DFP-3: Internal DisplayPort
(II) NVIDIA(0): Setting mode
(II) NVIDIA(0): "DFP-0:nvidia-auto-select+0+0,DFP-3:1920x1200+1440+0"
Wow, I've never seen that from a DisplayPort device! You say it actually lit up at 640x480? I'm very surprised that an EDID couldn't be read but the DPCD could. If there were some problem that prevented the driver from reading the EDID, I would have expected it to fail link training as well. Does this panel work in Windows? You might try contacting HP to see if it needs a firmware upgrade.
As for forcing the resolution, you might be able to do it by using the ExactModeTimingsDVI option and then setting the traditional HorizSync and VertRefresh options in the monitor section, but you shouldn't have to; an EDID is required by the DisplayPort spec so either the monitor is out of spec or there's a bug in the driver that's preventing it from reading it. I'll try to find one of these monitors to see if I can reproduce the problem.
Given that VGA works, another option might be to save the EDID using nvidia-settings and then use the CustomEDID option to make the driver read the EDID for DFP-3 from a file instead of trying to read it from the monitor.