All policy code in the kernel will be written in Scheme. There will be support for multiple APIs, and it will be possible to replace individual pieces of the system on the fly and, within the constraints of security, by ordinary users.
For information about some of the ideas we're considering see here or here. I've also put together a little bit on the ancient history of the project.
If you want to try running a scheme interpreter directly on a i386+ box, try this. Either from a floppy, or make lilo.conf point at it. We'll have a tiny emacs and file-system support very shortly.
Somewhat related efforts by other people are Ocelot and NASOS. There is a mailing list about Scheme-OS's in general at scheme_os-request@lists.sonic.net.
Neal Stephenson, author of SnowCrash, has written an interesting essay on operating system philosopy.
If you're in the Toronto area and interested, our meeting times are available here.