Here I have chosen to use a separate zone file for my subdomain. This is not the only way to do this, but it is the way I did it. I did this in CentOs 5.x.

One using it own zone file:
vi /var/named/chroot/etc/named.conf
…
zone “mydomain.com” in {
type master;
file “mydomain.com.zone”;
allow-transfer { xxx.yyy.zzz.aaa ; bbb.ccc.ddd.eee ; };
};

// Begin MyDomain.com Subdomains
zone “subdomain.mydomain.com” in {
type master;
file “subdomain.mydomain.com.zone”;
allow-transfer { xxx.yyy.zzz.aaa ; bbb.ccc.ddd.eee; };
};
// End MyDomain.com Subdomains
…

vi /var/named/chroot/var/named/subdomain.mydomain.com.zone
$ORIGIN .
$TTL 86400 ; 1 day
subdomain.mydomain.com IN SOA ns1.subdomain.mydomain.com. root.localhost.subdomain.mydomain.com. (
201002224 ; serial
7200 ; refresh (8 hours)
7200 ; retry (2 hours)
604800 ; expire (1 week)
86400 ; minimum (1 day)
)

$ORIGIN subdomain.mydomain.com.
NS ns1.subdomain.mydomain.com.
NS ns2.subdomain.mydomain.com.
IN MX 10 mail1.subdomain.mydomain.com.
IN MX 20 mail2.subdomain.mydomain.com.
IN A xxx.yyy.zzz.aaa
ns1 IN A xxx.yyy.zzz.aaa
ns2 IN A bbb.ccc.ddd.eee
mail1 IN A xxx.yyy.zzz.aaa
mail2 IN A bbb.ccc.ddd.eee
host1 IN A xxx.yyy.zzz.aaa
host2 IN CNAME host1.subdomain.mydomain.com.