In defense of my new favorite platform:
Firstly, it's about a 3 minute job to install new memory on an iBook. I added it to my TiBook in class while waiting for the teacher to start his lesson! (and he wasn't late).
Secondly, adding other upgrades (like the disk, etc), won't void your warranty unless it's actually your fault. Apple support is pretty cool in this regard. For example, if your LCD dies, it's covered under warranty even if you've replaced your HDD.
But, my experiences with Apple are from the US, so YMMV.