I am now able, in a limited way, to use my external Samsung SyncMaster 215TW with Ubuntu 8.10 running on laptop, using ATI Radeon Mobility UI.

To activate the external monitor:

• Turn on the external monitor.
• System / Preferences / Screen Resolution.
• First time only: Check ‘Show Displays in Panel’.  This means you can just click on the panel icon in future instead of using System / Preferences / Screen Resolution.
• First time only: Uncheck ‘Mirror Screens’ if it is checked
• Click on Samsung 21″, and set Resolution to the maximum 1680 x 1050.  Setting a lower resolution causes the external monitor to display only a warning message.
• IMPORTANT: Click on Laptop 15″, and set Resolution to Off.
• Click on Apply.

To switch back to the laptop screen, do the reverse.  This may be important if your next use of the laptop is going to be without the monitor.

If at any point, you end up with no display on either laptop or monitor, restart in recovery mode and execute:
> rm ~/.config/monitors.xml