I worked at Apple in the BSD group.

The XNU non-Mach code was clearly some BSD kernel and I don't really
care which.  My colleagues told me it started out with NetBSD but that
that was eventually dwarfed by FreeBSD with contributions from
elsewhere.  While I was there, there was talk of dragging in code from
the latest FreeBSD, notably the FFS with soft updates; I'm pretty sure
that happened.  Given that the group was (and probably still is)
headed by Jordan Hubbard of FreeBSD fame, I suspect that they're
continuing to pull in FreeBSD code and it isn't just hype.

Note too that the XNU BSD code, measured in source lines, is almost
exactly as huge as the Mach code, so the volume of *BSD code in XNU is
not, in my opinion, exaggerated: it is (or was in 2002) half the
kernel source.  (I don't remember which side of the fence IOKit was
counted against.)

Yes, the XNU kernel details are different from a stock BSD kernel.  It
co-exists with Mach, after all.  Porting graphical applications to
native OS X (avoiding X11) is a pain too; Apple do a lot of things
their own way, inheriting baggage from the pre-Unix Mac OS and
NextStep (netinfo is just the French spelling of `Yellow Pages', ugh).

Nevertheless, I stand by my statement that OS X is in no sense a
micro-kernel, and that user-mode file servers will not (as a result of
access to a micro-kernel) be easier to implement on OS X than on other
(l)unixes.

However, Martin Atkins has revealed the mystery kernel agent: coda.
Apparently it's somewhat specialised but lets user-mode file servers
catch opens and closes.

Anyone in (l)unixland for a filesystem switch?  Research Unix had one
~20 years ago, so it should be mouldy (er, mature) enough to be
acceptable to (l)unixland.  Throw in mounts by ordinary users and use
of 9P as an unifying filesystem protocol (now pretty well aged in Plan
9), and it becomes possible to push lots of code and some hacks out of
the kernel, while permitting some new and interesting work.