I recently upgraded to Ubuntu 8.10, and everything works really well apart from the dual monitor support.
My laptop has an NVidia graphics chip set. The laptop screen works well, but the attached second monitor was blank and did not show up in the display configuration applet (System > Preferences > Screen Resolution).
After installing the proprietary NVidia driver (required when I selected "Extra" level visual effects from the Appearance applet (System > Preferences > Appearance) the fix is relatively straight forward, a slight tweak to the
/etc/X11/xorg.conf file, changing the the "Device" section to look like this:
Section "Device" Identifier "Configured Video Device" Driver "nvidia" Option "NoLogo" "True" ## JohnE added the following options for dual monitor support Option "TwinView" "True" Option "TwinViewOrientation" "RightOf" Option "UseDisplayDevice" "CRT,DFP" EndSection