Here is a list of patches that activate the dual display on various iBook, eMac and iMac. Try it at your own risks. If it doesn't work, you can reset the pRAM with option-command-p-r, hold those keys at startup, and wait for the computer to restart 3 time.
list of compatible mac:
http://www.rutemoeller.com/mp/ibook/supportlist_e.html
software page:
http://www.rutemoeller.com/mp/ibook/ibook_e.html
Imac G5 (the page is in German only):
http://www.heise.de/ct/04/23/links/020.shtml
JC