General Remarks
Ok, why choosing an iBook2 as your notebook (or desktop-replacement) ? It might not be the fastest computer (see performance) and it also might not be the cheapest one but there are good reasons for it too.
When I'm buying things for my computer I always look at the noise they produce. Thats why I ended up with a SMP-Pentium3 instead of a fancy Athlon or Pentium4. These new CPUs produce a whole lot of heat and you have some trouble cooling it. My P3s have some decent coolers where the fans run at 7 Volts, so they are noise-less (for all practical purposes). If you want to buy a notebook things get even more complicated because you can not cool your prcessor so efficiently as in a tower case. So you don't have many choices: a MobileP3 or a PowerPC. The G3 in the iBook uses only around 6 Watts in average (correct my if I'm wrong). So one point for the iBook.
Of course you want to run Linux on your baby (at least I want ;-). So you are interested in good drivers for your machine. In the PC-World there are a lot of different notebooks and each of the manufacturer uses different chips and it's not always easy to find documentation for developing drivers. But there are only two Apple notebooks and they are really good documented (AFAIK). In fact my Linux supports (almost) all of the features of my iBook.
At first I wanted the iBook-600 MHz with the DVD-ROM-drive. But unfortunatly all they had was the version with the DVD/CD-RW combo-drive, the 20 GB hdd. If your are looking for an equally equipped (and equally small) notebook in the PC-world you might have some trouble getting one for the same price. So the iBook is also not that expensive.
And of course: It looks cool . It's small, has a really beautiful packaging, this cool illuminated Apple-logo and it runs MacOS-X (Ok, not _that_ important if you have Linux ;-).
So in the end, there are many points speaking for the iBook.