Been spending time studying things like Plan 9 and understanding the strengths of its design on some issues such as namespaces and thinking about how best to do it in Linux. Al Viro did a private namespaces in Linux hooking into the "clone" system call with additional flags. On FreeBSD this could be done in rfork() and on Mac OS X we don't have either of those so we'd have to find an appropriate mach_task setting or implement rfork to do something similar.
Private namespaces are very cool and can do away with such things as symbolic links and jails/chroots if implemented properly. DragonFlyBSD has an implementation of something called "variant symlinks" in which a special environemnt variable can complete a portion of a path for a symbolic link. /usr/lib/libqt.VAR.so could point to many different versions of qt based on an environment variable. Pretty neat stuff.
Not sure how far I'll get trying to implement stuff like that in xnu [darwin/Mac OS X kernel] but it's just for fun right now anyway.
I'm going to repartition my Firewire drive and get a bootable Panther installation on there for playing around.