I think the culprit of my spinnies on OS X problem was Java.
I had installed Developer Preview 10 of jdk 1.4.1, and I run jEdit as my editor of choice. If it crashed, when I tried to start it again the iBook went into spinnies mode.
It's strange, as jEdit was actually running under JDK 1.3 (that's Apples backward compatibility strategy - keep two JDK's installed and when java apps run if they don't specify a JDK version then use the older one). I've switched it to running under jdk 1.4 now, and although I have some strange artifacts (lost buttons and odd looking gui elements) it's a lot faster, and I'll see if I get the same bug (hopefully it'll actually be more stable).