IBM developerWorks has a tutorial
(registration required) on building a custom embedded Linux distribution.
"This tutorial shows you how to install Linux on a target system. Not
a prebuilt Linux distribution, but your own, built from scratch. While the
details of the procedure necessarily vary from one target to another, the
same general principles apply. The result of this tutorial (if you have a
suitable target) is a functional Linux system you can get a shell prompt
to post comments)