there is a (still) small group ('cuby' and myself) starting to work on porting the Apple Darwin Microkernel (also called XNU) to the Sharp Zaurus. The ideas behind are
* provide a BSD compatible real microkernel
* learn something
* a microkernel is something the Embedded World might look for (especially interesting for single-processor mobile phones)
* Darwin is very robust
* Development tools are for free (well, you need a Mac and a Zaurus - but development can be done using free Xcode & gcc)
So, if you are interested, please have a look at a first description page I have put on my server. And there is a forum for further technical discussions. It already contains some materials on how to create a gcc-3.4.2 cross compiler, the Mach-O format, what Darwin is, and how it works internally.
Here are the links: