If there is no name server available at boot time (i.e. because the
ypclient in question is the name server, and BIND starts after ypbind)
ypbind will fail to bind if the server is specified in /etc/yp.conf a la:

  domain ypdomain server ypserver.mydomain.com

Or, to be more specific, ypbind WILL NOT use /etc/hosts to determine the
hostname of the server specified in /etc/yp.conf and therefore if there is
no dns server available, the ypbind will fail.

One workaround is to change the start order of named to preceed ypbind.
IIRC, I was able to use the server's IP address in place of its name, and
that did work, but I don't remember for certain now.

This is ypbind-3.3-24, which ships with RH6.1 -- I did not see any updates
for this package in the updates directory.