I had some trouble getting my laptop (Dell Precision M65) to work using nvidia's TwinView. I wanted it to automatically use my CRT monitor @ 1280x1024 if docked but to use the laptop panel @ 1920x1200 if not docked.
Here's my xorg.conf after I figured out how to do it, all this is documented in Nvidia's README but it's a bit confusing without an example.
The key points are that there are two "Screen"s and that the Twinview and Metamodes options are needed in the "Device" section. In my case the Usedisplaydevice options in the Screen sections are also required otherwise it can choose the wrong one. That's why these intructions 
http://javier.rodriguez.org.mx/index.php/2006/09/08/xorgconf-configuration-for-nvidia-twinview-on-the-dell-620/
didn't work for me.
Section "ServerLayout"
Identifier "Default Layout"
Screen 0 "Docked"
Screen 1 "Laptop"
InputDevice "Generic Keyboard"
InputDevice "Docked Mouse"
InputDevice "Configured Mouse"
InputDevice "stylus" "SendCoreEvents"
InputDevice "cursor" "SendCoreEvents"
InputDevice "eraser" "SendCoreEvents"
InputDevice "Synaptics Touchpad"
EndSection
Section "Files"
# path to defoma fonts
FontPath "/usr/share/X11/fonts/misc"
FontPath "/usr/share/X11/fonts/cyrillic"
FontPath "/usr/share/X11/fonts/100dpi/:unscaled"
FontPath "/usr/share/X11/fonts/75dpi/:unscaled"
FontPath "/usr/share/X11/fonts/Type1"
FontPath "/usr/share/X11/fonts/100dpi"
FontPath "/usr/share/X11/fonts/75dpi"
FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection
Section "Module"
Load "bitmap"
Load "ddc"
Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "type1"
Load "vbe"
EndSection
Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "CoreKeyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc104"
Option "XkbLayout" "us"
EndSection
Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/input/mice"
Option "Protocol" "ExplorerPS/2"
Option "ZAxisMapping" "4 5"
EndSection
Section "InputDevice"
Identifier "Docked Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/psaux"
Option "Protocol" "ExplorerPS/2"
Option "ZAxisMapping" "4 5"
EndSection
Section "InputDevice"
Identifier "Synaptics Touchpad"
Driver "synaptics"
Option "SendCoreEvents" "true"
Option "Device" "/dev/psaux"
Option "Protocol" "auto-dev"
Option "ZAxisMapping" "4 5"
Option "HorizScrollDelta" "0"
EndSection
Section "InputDevice"
# /dev/input/event
# for USB
Identifier "stylus"
Driver "wacom"
Option "Device" "/dev/wacom" # Change to
Option "Type" "stylus"
Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection
Section "InputDevice"
# /dev/input/event
# for USB
Identifier "eraser"
Driver "wacom"
Option "Device" "/dev/wacom" # Change to
Option "Type" "eraser"
Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection
Section "InputDevice"
# /dev/input/event
# for USB
Identifier "cursor"
Driver "wacom"
Option "Device" "/dev/wacom" # Change to
Option "Type" "cursor"
Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection
Section "Monitor"
Identifier "Laptop Panel"
HorizSync 30.0 - 75.0
VertRefresh 30.0 - 60.0
#DisplaySize 334 210
Option "DPMS"
EndSection
Section "Monitor"
Identifier "ExternalCRT"
VendorName "Dell"
ModelName "Dell D1226H"
HorizSync 30.0 - 95.0
VertRefresh 50.0 - 160.0
Gamma 1
#DisplaySize 360 270
Option "DPMS"
ModeLine "640x480@60" 25.2 640 656 752 800 480 490 492 525 -hsync -vsync
ModeLine "640x480@72" 31.5 640 664 704 832 480 489 491 520 -hsync -vsync
ModeLine "640x480@75" 31.5 640 656 720 840 480 481 484 500 -hsync -vsync
ModeLine "640x480@85" 36.0 640 696 752 832 480 481 484 509 -hsync -vsync
ModeLine "800x600@56" 36.0 800 824 896 1024 600 601 603 625 +hsync +vsync
ModeLine "800x600@72" 50.0 800 856 976 1040 600 637 643 666 +hsync +vsync
ModeLine "800x600@75" 49.5 800 816 896 1056 600 601 604 625 +hsync +vsync
ModeLine "800x600@85" 56.3 800 832 896 1048 600 601 604 631 +hsync +vsync
ModeLine "800x600@60" 40.0 800 840 968 1056 600 601 605 628 +hsync +vsync
ModeLine "832x624@75" 57.3 832 864 928 1152 624 625 628 667 -hsync -vsync
ModeLine "1024x768@85" 94.5 1024 1072 1168 1376 768 769 772 808 +hsync +vsync
ModeLine "1024x768@75" 78.8 1024 1040 1136 1312 768 769 772 800 +hsync +vsync
ModeLine "1024x768@70" 75.0 1024 1048 1184 1328 768 771 777 806 -hsync -vsync
ModeLine "1024x768@60" 65.0 1024 1048 1184 1344 768 771 777 806 -hsync -vsync
ModeLine "1024x768@43" 44.9 1024 1032 1208 1264 768 768 776 817 +hsync +vsync interlace
ModeLine "1152x864@75" 108.0 1152 1216 1344 1600 864 865 868 900 +hsync +vsync
ModeLine "1152x768@54" 65.0 1152 1178 1314 1472 768 771 777 806 +hsync +vsync
ModeLine "1280x854" 80.0 1280 1309 1460 1636 854 857 864 896 +hsync +vsync
ModeLine "1280x1024@75" 135.0 1280 1296 1440 1688 1024 1025 1028 1066 +hsync +vsync
ModeLine "1280x960@60" 102.1 1280 1360 1496 1712 960 961 964 994 -hsync +vsync
ModeLine "1280x960@85" 148.5 1280 1344 1504 1728 960 961 964 1011 +hsync +vsync
ModeLine "1280x1024@85" 157.5 1280 1344 1504 1728 1024 1025 1028 1072 +hsync +vsync
ModeLine "1280x1024@60" 108.0 1280 1328 1440 1688 1024 1025 1028 1066 +hsync +vsync
ModeLine "1280x960@75" 129.9 1280 1368 1504 1728 960 961 964 1002 -hsync +vsync
ModeLine "1400x1050@60" 122.6 1400 1488 1640 1880 1050 1051 1054 1087 -hsync +vsync
ModeLine "1400x1050@75" 155.8 1400 1496 1648 1896 1050 1051 1054 1096 -hsync +vsync
ModeLine "1600x1200@65" 175.5 1600 1664 1856 2160 1200 1201 1204 1250 +hsync +vsync
ModeLine "1600x1200@60" 162.0 1600 1664 1856 2160 1200 1201 1204 1250 +hsync +vsync
ModeLine "1600x1200@75" 202.5 1600 1664 1856 2160 1200 1201 1204 1250 +hsync +vsync
ModeLine "1600x1200@70" 189.0 1600 1664 1856 2160 1200 1201 1204 1250 +hsync +vsync
ModeLine "1792x1344@60" 204.8 1792 1920 2120 2448 1344 1345 1348 1394 -hsync +vsync
ModeLine "1856x1392@60" 218.3 1856 1952 2176 2528 1392 1393 1396 1439 -hsync +vsync
ModeLine "1920x1440@60" 234.0 1920 2048 2256 2600 1440 1441 1444 1500 -hsync +vsync
ModeLine "2048x1536@60" 266.9 2048 2200 2424 2800 1536 1537 1540 1589 -hsync +vsync
EndSection
Section "Device"
Identifier "NVIDIA 0"
Driver "nvidia"
BusID "PCI:1:0:0"
Option "TwinView" "On"
Option "MetaModes" "DFP-0: 1920x1200, CRT-0: NULL; DFP-0: NULL, CRT-0: 1280x1024"
#Option "MetaModes" "DFP-0: 1920x1200, CRT-0: 1280x1024; DFP-0: 1920x1200, CRT-0: NULL; DFP-0: NULL, CRT-0: 1280x1024"
#Option "HorizSync" "DFP-0: 30-75; CRT-0: 30-95"
#Option "VertRefresh" "DFP-0: 30-60; CRT-0: 50-160"
#Option "SecondMonitorHorizSync" "30-75"
#Option "SecondMonitorVertRefresh" "30-60"
#Option "TwinViewOrientation" "Leftof"
#Option "ConnectedMonitor" "DFP, CRT"
#Option "UseEdidDpi" "False"
EndSection
Section "Screen"
Identifier "Laptop"
Device "NVIDIA 0"
Monitor "Laptop Panel"
Option "UseDisplayDevice" "DFP-0"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1920x1200" "1680x1050" "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1920x1200" "1680x1050" "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection " Display"
Depth 8
Modes "1920x1200" "1680x1050" "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1920x1200" "1680x1050" "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1920x1200" "1680x1050" "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1920x1200" "1680x1050" "1440x900" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Section "Screen"
Identifier "Docked"
Device "NVIDIA 0"
Monitor "ExternalCRT"
Option "UseDisplayDevice" "CRT-0"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1600x1200@75" "1280x1024@85" "1280x1024@60" "1280x960@85" "1280x960@75" "1280x960@60" "1400x1050@60" "1280x1024@75" "1400x1050@75" "1280x854" "1600x1200@65" "1152x768@54" "1600x1200@60" "1152x864@75" "1600x1200@75" "1024x768@43" "1600x1200@70" "1024x768@60" "1792x1344@60" "1024x768@70" "1856x1392@60" "1024x768@75" "1920x1440@60" "1024x768@85" "2048x1536@60" "832x624@75" "800x600@60" "800x600@85" "800x600@75" "800x600@72" "800x600@56" "640x480@85" "640x480@75" "640x480@72" "640x480@60"
EndSubSection
EndSection
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