Robert, I've had my G4 933 iBook for about a 3 weeks now and love it. My work machine (I'm an illustrator) is a G4 867 with 640mb of ram. The ibook has 256mb.
Now, I don't use the iBook for my work, so I can't give you a direct comparison, my main software is Corel's Painter 8, which is not loaded on the iBook. General impressions are that the iBook is just a tad slower than my work machine doing basic things like opening & closing files, etc. Goofing around in Photoshop and Illustrator the speeds seem comparable, but then again, I have no graphics tablet hooked up to the ibook and haven't played around with large files. As Chealion pointed out, the hard drive could be the speed difference and obviously I have double the ram in the G4 tower.
If I get some time in the next few days, I'll load Painter on the iBook and hook up the Wacom tablet and put it through its paces. I'll get back to this post, I'm kind of curious myself on the comparison.