/var/log/message show below~
Dec 23 09:55:02 mail named[1108]: error (network unreachable) resolving 'pdns5.ultradns.info/A/IN': 2001:500:19::1#53
Dec 23 09:55:02 mail named[1108]: error (network unreachable) resolving 'pdns5.ultradns.info/AAAA/IN': 2001:500:1a::1#53
Dec 23 09:55:02 mail named[1108]: error (network unreachable) resolving 'pdns5.ultradns.info/A/IN': 2001:500:1b::1#53

Workaround / Solution :
Disable the IPV6 in BIND. For CentOS 5.4, edit the /etc/sysconfig/named file and add the following options into the BIND startup

OPTIONS=”-4″

This will cause the BIND server to only resolve or use IPV4 and disable IPV6 support. Save the file and restart BIND server