mod_asn is used in production with several hundred requests per seconds since January 2009. It's feature complete and stable.
There were no security incidents.
mod_asn supports IPv4, as the BGP routing table of IPv4 autonomous systems does. IPv6 cannot be supported with this routing table. There are IPv6 autonomous systems already, in lesser quantity. mod_asn won't be able to use these because the underlying technology, postgresql-ip4r, does work only with IPv4 IP addresses. The developers started working on IPv6 support, see http://archives.postgresql.org/pgsql-committers/2011-08/msg00016.php
This link should guide you to open mod_asn bug reports in the issue tracker.