thanks to geoff for pointing me in the correct direction.

the rfc in question is rfc 2317. i'll outline what's required,
since it wasn't obvious to me.

here's an example setting up t2317 ptr records for 65.14.39.130 and
65.14.39.131 in the network 64.14.39.128/123.

# first we have to have the correct ipnet setup.
# dns needs this information to find the netmask.

ipnet=coraidt1 ip=65.14.39.128 ipmask=/123
# other stuff you may need goes here.

# next we need to declare our dom. dns normally does this
# for us, but for 2317-style reverse lookups, we do it ourselves.

dom=128.39.14.65.in-addr.arpa soa=
refresh=3600 ttl=3600
ns=ns1.coraid.com
ns=ns2.coraid.com

# and the entries.
dom=130.128.39.14.65.in-addr.arpa
dom=131.128.39.14.65.in-addr.arpa

- erik