On Ubuntu 9.10, where is the xorg.conf file in Karmic Koala?

xorg.conf will not be present unless it is explicitly required.

Wikipedia on xorg.conf:

For a long time, editing xorg.conf was necessary for advanced input devices and multiple monitor output to work correctly. This was regarded to be a major usability obstacle. In modern systems this is seldom necessary, thanks to input hotplugging and the XRandR extension integrated into new X.org releases. It is still needed for devices from some manufacturers, notably NVIDIA and Wacom, whose drivers fail to provide support for those technologies.

You should investigate if you can use the RandR extension to change the display characteristics. xrandr provides the command-line interface to this extension. From the man page:

Xrandr  is  used  to set the size, orientation and/or reflection of the
outputs for a screen. It can also set the screen size.

If invoked without any option, it will dump the state of  the  outputs,
showing  the existing modes for each of them, with a ’+’ after the pre‐
ferred mode and a ’*’ after the current mode.

The new X server doesn't need an xorg.conf file anymore. Rather, it is gathering the devices with D-Bus and may be installed without it on Jaunty. You can still build your own though if you need to add something.

Source: Where is my xorg.conf file??