In the interest of clarity, I have put together the following, what
worked for me. Of course, it's been a while since I did this, so I'm not
making any iron-clad promises here.

Okay - a brief summary of how to netboot, as it is working for me:
Note - make sure you replace all ethernet and IP addresses and hostnames
with the ones appropriate for your situation. This is assuming a private
network based on the 192.168.0.x class C, where the router and boot
server is 192.168.0.1, and the client computer is 192.168.0.2

On the Boot Server, set up rarpd, mopd, and dhcpd to run at boot-time,
using the sample dhcpd.conf below:

Start /etc/dhcpd.conf ---------------------------------------
subnet 192.168.0.0 netmask 255.255.255.0 {
        option domain-name-servers 192.168.0.1;
        option subnet-mask 255.255.255.0;
        option routers 192.168.0.1;
        range 192.168.0.128 192.168.0.255;
}

host vs4060 {
        hardware ethernet 08:00:2b:0e:c2:ea;
        fixed-address 192.168.0.2;
        option root-path "/usr/export/bootclient";
}
End /etc/dhcpd.conf ---------------------------------------

Set up nfsd and mountd to load at boot-time, using the sample exports
file below:
Start /etc/exports --------------------------------
/usr/export -alldirs -maproot=root
End /etc/exports --------------------------------

Create a directory, /usr/export/bootclient and unpack the NetBSD distro
into it, including the kernel. Create a directory, /tftpboot/mop, and
copy the appropriate MOPBOOT.SYS into it, and make a symbolic link to
that file as such: 08002b0ec2ea.SYS - note the capitol SYS (and of
course, use the correct ethernet address). Make sure that the ethernet
address is listed in the /etc/ethers with the appropriate hostname, and
that the hostname is listed in /etc/hosts with the appropriate IP
address. Note also that in /etc/ethers, the ethernet address should NOT
include leading zeros in each octect - ex: 8:0:2b:e:c2:ea. Sample
/etc/hosts and /etc/ethers below:

Start /etc/hosts -------------------------------
192.168.0.1     bootserver
192.168.0.2     bootclient
End /etc/hosts -------------------------------

Start /etc/ethers ------------------------------
0:c0:f0:19:50:a1  bootserver
8:0:2b:e:c2:ea  bootclient
End /etc/ethers ------------------------------

Once this stuff is all in place, reboot the boot server (easy way) or
start rarpd, mopd, dhcpd, nfsd, and mountd (hard way). Hop on over to
your client machine and boot it up. At the chevron prompt, type 'b
xqa0', or 'b esa0', whatever is appropriate to your hardware. If all
goes well, your server should boot up into single user mode all on it's
own at this point.

Let me know if this helped.
--
 -Buckaroo