This is the case that I have been using on my iBook G3/900 12" for the last six months...
http://eshop.macsales.com/Catalog_It...Item=OWCIBAG12
it's inexpensive, and very simple...black nylon, with a "fuzzy", soft interior that does not mar the iBook at all...it's a play-thru case, meaning I don't have to take my computer out to use it...just unzip, open the iBook - and you're ready to go!
One nice touch - the zippers even have a felt backing, so the metal does not scratch the iBook...
I have been VERY happy with this case, because it is perfect to carry the computer and nothing else...when I need to carry everything, I just slide the OWC case into my bigger Kensington bag...fits like a glove, and the handles collapse so they take up no room...
I found this case kinda by accident...I always buy my memory and hard drive upgrades from OWC (their service is excellent!), and when I was ordering my RAM chip, I just happened to see this case...It looks a bit homemade, but it is the best play-thru solution I have found, and at $29.95 - it's a steal!
CK
iBook G3/900 12"
640MB / 40GB / Airport