Ah-ha, the usual! Get TWO people to look at a problem and it goes away! Currently, as I type kernel_task is back down where it ought to be about 1% CPU or so. It's doing this for spite I know it!
Before that I did a Safe Boot which did the same thing, 100% CPU usage on kernel_task
I shutdown, removed an unused SCSI card for grins. This had been installed without incident for a long time, though. I dunno, it was something to do.
Then I booted normally, got a kernel panic just as it started to finish booting.

Booted again, worked okay. Now kernel_task is behaving itself. Weird. Spotlight is not indexing. I dunno, might just be time for a good reinstall.