Unfortunately, it seems like it is not possible to use an external screen unless it was plugged in at the point that X was last run. Therefore, either plug your screen in before you turn on your EEE, or plug it in then restart X by pressing Ctrl+Alt+Backspace.
You can install the 'resapplet' to change resolutions, it seems light and good at its job. It runs as an icon in the tray.
sudo apt-get install resapplet
If you wish to close your EEE while using an external screen, and find the external screen turns off, just wiggle the mouse and click, it will come back to life.
Everything is tiny and unreadable (I found) on my 17 inch screen at 1024×768, I presume this is due to the font changes for the small screen. To correct this, navigate to the applications menu > Settings > Settings Manager > User Interface. Increase the font size to say 14. Your choice. I found this did not take effect on the desktop until I right clicked the desktop, then Desktop Settings, then disabled and re-enabled 'Use system font size' in the Behaviour tab.
Everybody feel free to add to and edit this, it was a brief and not particularly well-planned effort, but I hope the info helps.
Some of the info here came from the forums, many thanks to the original posters!