The German website macparts.de/iBook has posted instructions (in both German and English) to enable monitor spanning on the iBook in OS X. With a few firmware commands, one can be enjoying two seperate workspaces with an iBook and an external monitor. I'm using it right now and it works great!
[Editor's note: Please read the cautions on the instructions page before proceeding! I have not tried this one, and probably won't because I tend to be overly cautious about modifying hardware...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021024062517608