It occurred to me that Daniel Bolgheroni wrote in gmane.os.netbsd.help:
> Hi,
>
> I have a testing machine running NetBSD-2.0, Fedora Core 3 and Windows
> XP Pro SP2, multibooting with grub. But everytime I boot, NetBSD gets
> the hostname of the last OS booted (Fedora or Windows), not the hostname
> specified in /etc/rc.conf.

The others probably report their hostname back to the DHCP server, who 
sends it back when NetBSD requests an address.

Unfortunately, you can't tell dhclient not to touch the hostname as far
as I know, but what you could do is adding something like

supersede host-name "my.desired.hostname";

to your /etc/dhclient.conf