Summary

An alternative way to perform reverse lookup

Friendly with scoped addresses
Works even for link-locals
Site-local - see the other discussion...

Some behavior differences with PTR

Easy to implement
Responder: KAME/USAGI kernel has it in ICMPv6 processing
Querier: KAME revlookupd, ping6 -w

Shouldn't trust reverse mapping too much anyways, btw
draft-ietf-dnsop-inaddr-required-03.txt