In light of World IPv6 Launch day, I would like to raise awareness for RFC5549, "Advertising IPv4 Network Layer Reachability Information with an IPv6 Next Hop" (http://tools.ietf.org/html/rfc5549).
As far as I'm aware of, this RFC has yet to be implemented by any vendors so far but it could help massively in the increasing scenarios where IPv4 addressing is scarce. This includes
many IXPs which invariably will sooner or later run into this problem.
If you happen to be attending the last day of NANOG, I'd be happy to talk to you in person about this for further details (I'm the dude with "RFC5549" written on the back of his laptop).