I have a dell Inspiron 640m, which works awesome with Linux... I can use the internal and external screens under windows easily and perfectly though I don't use windows very offen. If I close the lid on my laptop and boot it, the external screen becomes the main screen and everything works fine under linux using only the external screen. The only way I've been able to use the internal screen is to boot with the laptop open, then, the internal screen is the only screen that works. Never both under linux. The only thing I've changed in effort to use both screens at once is the X config file. With the xorg.conf below, X works as explained above.

# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the /etc/X11/xorg.conf manual page.
# (Type "man /etc/X11/xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
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"
# path to defoma fonts
FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection

Section "Module"
Load "bitmap"
Load "ddc"
Load "dri"
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"
Option "Emulate3Buttons" "true"
EndSection

Section "InputDevice"
Identifier "Synaptics Touchpad"
Driver "synaptics"
Option "SendCoreEvents" "true"
Option "Device" "/dev/psaux"
Option "Protocol" "auto-dev"
Option "HorizScrollDelta" "0"
EndSection

Section "Device"
Identifier "Intel"
Driver "i810"
BusID "PCI:0:2:0"
# Screen 0
EndSection

Section "Device"
Identifier "Intel2"
Driver "i810"
BusID "PCI:0:2:0"
# Screen 1
EndSection

Section "Monitor"
Identifier "Laptop Monitor"
Option "DPMS"
# HorizSync 28-72
# VertRefresh 43-60
EndSection

Section "Monitor"
Identifier "External Monitor"
Option "DPMS"
# HorizSync 30-65
# VertRefresh 50-75
EndSection

Section "Screen"
Identifier "Laptop Screen"
Device "Intel"
Monitor "Laptop Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1440x900"
EndSubSection
SubSection "Display"
Depth 4
Modes "1440x900"
EndSubSection
SubSection "Display"
Depth 8
Modes "1440x900"
EndSubSection
SubSection "Display"
Depth 15
Modes "1440x900"
EndSubSection
SubSection "Display"
Depth 16
Modes "1440x900"
EndSubSection
SubSection "Display"
Depth 24
Modes "1440x900"
EndSubSection
EndSection

Section "Screen"
Identifier "External Screen"
Device "Intel2"
Monitor "External Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024"
EndSubSection
EndSection

Section "ServerLayout"
Identifier "Dual Head Layout"
Screen "Laptop Screen" 0 0
Screen "External Screen" RightOf "Laptop Screen"
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
InputDevice "Synaptics Touchpad"
EndSection

Section "DRI"
Mode 0666
EndSection

Section "ServerFlags"
Option "Xinerama" "enable"
EndSection



lspci
0000:00:00.0 Host bridge: Intel Corporation Mobile Memory Controller Hub (rev 03)
0000:00:02.0 VGA compatible controller: Intel Corporation Mobile Integrated Graphics Controller (rev 03)
0000:00:02.1 Display controller: Intel Corporation Mobile Integrated Graphics Controller (rev 03)
0000:00:1b.0 0403: Intel Corporation 82801G (ICH7 Family) High Definition Audio Controller (rev 01)
0000:00:1c.0 PCI bridge: Intel Corporation 82801G (ICH7 Family) PCI Express Port 1 (rev 01)
0000:00:1c.1 PCI bridge: Intel Corporation 82801G (ICH7 Family) PCI Express Port 2 (rev 01)
0000:00:1c.3 PCI bridge: Intel Corporation 82801G (ICH7 Family) PCI Express Port 4 (rev 01)
0000:00:1d.0 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI #1 (rev 01)
0000:00:1d.1 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI #2 (rev 01)
0000:00:1d.2 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI #3 (rev 01)
0000:00:1d.3 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI #4 (rev 01)
0000:00:1d.7 USB Controller: Intel Corporation 82801G (ICH7 Family) USB2 EHCI Controller (rev 01)
0000:00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev e1)
0000:00:1f.0 ISA bridge: Intel Corporation 82801GBM (ICH7-M) LPC Interface Bridge (rev 01)
0000:00:1f.2 IDE interface: Intel Corporation 82801GBM/GHM (ICH7 Family) Serial ATA Storage Controllers cc=IDE (rev 01)
0000:00:1f.3 SMBus: Intel Corporation 82801G (ICH7 Family) SMBus Controller (rev 01)
0000:02:00.0 Ethernet controller: Broadcom Corporation BCM4401-B0 100Base-TX (rev 02)
0000:02:01.0 FireWire (IEEE 1394): Ricoh Co Ltd: Unknown device 0832
0000:02:01.1 0805: Ricoh Co Ltd R5C822 SD/SDIO/MMC/MS/MSPro Host Adapter (rev 19)
0000:02:01.2 System peripheral: Ricoh Co Ltd: Unknown device 0843 (rev 01)
0000:02:01.3 System peripheral: Ricoh Co Ltd R5C592 Memory Stick Bus Host Adapter (rev 0a)
0000:02:01.4 System peripheral: Ricoh Co Ltd xD-Picture Card Controller (rev 05)
0000:0c:00.0 Network controller: Intel Corporation: Unknown device 4222 (rev 02)