I am working on a project which has NetBSD on a vendor branch with
signficant local changes.   I just imported -current (last Wednesday);
the previous import was from netbsd-2-0 in October.  I had only minor
merge issues (some constification, domains now using link sets, struct
domain changed a bit), and the new kernel worked just fine with the
old userland (i386, with ath(4)), and interoperated with 2.0_RC4
machines over ath(4) in IBSS mode.

I know that installing a new kernel and userland at the same time is
asking for trouble, and that one should boot a new kernel first, but
one can often get away with it (e.g. if none of the tools needed to
finish writing the new userland and reboot rely on new system calls),
and it's easier if it works (I have about 25 machines).  I tried my
normal upgrade script (install kernel and userland from sets,
automerge /etc, update packages from manifest and binary packages) and
it mostly succeeded, losing on statvfs1 in opendir.  But I was able to
reboot, and I then reran the upgrade.

All in all I was very happy with the ease of merging and the lack of
trouble - the machine I upgraded only has a wireless interface and I
didn't have to physically touch it.

-- 
        Greg Troxel <gdt@ir.bbn.com>