kernal_task 642 MB - what is it?

[Chakravartin]

Lots of components of the Mac OS will get grouped under that heading. They load as they are needed, so the kernel task may start out small after a restart and then take up more RAM and cpu time as you use your Mac.

More detail than you want:
http://reviews.cnet.com/8301-13727_7-200...m-in-os-x/