For a while now I've been working with Fred Sánchez on Darwin's SystemStarter project.
The initial version of SystemStarter was basically a replacement of the /etc/rc mechanism, with some neat forward-looking design decisions on Fred's part. I've found this area of operating systems to be traditionally weak. The infancy (and malleability) of Darwin, combined with the neat work Fred started, got me interested in the project. Since then, Fred and I have been working on more advanced features, some of which are unique to Darwin.
Overall I think it's pretty exciting stuff. SystemStarter is breaking away from an entrenched part of UNIX, and providing something more robust.
The SystemStarter work is discussed in the paper "SystemStarter and the Mac OS X Startup Process" which I had the opportunity to co- author along with Fred, and will be presented at BSDCon 2002. I'm really excited about BSDCon; it's the first time I've been published, and it helps to fuel that typical Apple- related delusion of working on software that makes a difference :-)
The ultimate goal is to allow services to be started, stopped, and restarted dynamically; and to have them communicate with SystemStarter enough that SystemStarter can make informed decisions.