OS X still has some fragility (or bugs?) in kernel-related code, it would seem. I just had my laptop go to 150% CPU usage … with no apps running. “top” showed that the rogue process was the core kernel process (PID 0) – i.e. only way to stop it is to reboot.
But I’ve noticed that some bugs related to the kernel can be “fixed” simply by hibernating and de-hibernating. e.g. with Wacom tablets, early versions of OS X 10.6 would often get “stuck” with the right-mouse-button permanently clicked. (IIRC, that got fixed around 10.6.3 – either by Apple or by Wacom)
So, I tried it today: close laptop lid, wait 1 second, re-open.
Magic! Kernel process un-f***ed itself, and system went back to normal. Many times simpler and quicker than rebooting :).
(EDIT: although … a few minutes later, I’m now seeing 0.5 second delays on keyboard interaction and mouse movement, every few seconds. Seems the OS X kernel is still FUBAR. Gah.)