I tried it, and here's how it went.
I used Disk Utility to put the 10.5.8 image from my eMac onto my iBook, the iBook previously had 10.4.11, which ran moderately slowly. The image transfer took around four hours, but it worked. Despite being sub-system requirement, Leopard will run on a 800MHz, 256MB RAM 12" iBook. As far as I can see, it has even adapted to the change in screen size, trackpad as to mouse, battery etc. In fact, it is as if the image was designed for the iBook.
However, the bad news is it is very slow, Spaces and Time Machine etc mean that a lot of the processing power is being used even when idle. To demonstrate, here are a few comparisons of how long it takes to open applications from idle:
eMac: 3.02 seconds
iBook: 16.21 seconds
eMac: 24.69 seconds
iBook: 46.67 seconds
eMac: 11.36 seconds
iBook: 31.58 seconds
I will enjoy having a portable version of my eMac, it's certainly more use to me than the tiger image it used to run. But, I might find myself waiting until I'm home at my eMac to do things than do them then and there on the iBook due to the agonising speeds. I'm considering buying more RAM but I'm wondering whether the amount I will use it and the actual increase of speed will justify the cost.