well, consider: reverse-lookups are mostly done when you connect somewhere, by the machine you connect to, asking the ptr of the ip you connect from. the machine most certainly has ipv6 connectivity, so an ipv6-only nameserver SHOULD work fine. but, as most organisations have central nameservers doing the resolving, which may or may not have ipv6 connectivity, i suggest using a nameserver which has both, ipv4 and ipv6 connectivity. also note, that some software may cache the previous NXDOMAIN answer and it may take up to 7 days before lookup is tried again.