Yesterday I set up a subdomain for an Isilon cluster -- they use the subdomain to handle load balancing across nodes. We were all surprised to discover that the forwarders option in named.conf) prevents subdomain delegation -- thensrecord for the subdomain was completely ignored. Once I removed theforwarders` clause, the subdomain worked.
Normally, forwarders in the options block of named.conf is handy -- it informs named of local resolvers, so named can get answers from a nearby/fast resolver, rather than going all the way up to the international root servers and working its way back down.