freebsd stable - Sep 2008 - IPV6_V6ONLY and UDP

I see in the release notes for 7.0; and seperately CURRENT on alpha;
that IPV6_V6ONLY is now fully supported for UDP.

Can someone point me to code/text that explains what exactly was broken 
about it? In what case(s) udp sockets set with IPV6_V6ONLY accepted V4 
packets?

Thanks in advance..

-- 
  Jason Fesler, email/jabber <jfesler@gigo.com> resume: http://jfesler.com
  "Give a man fire, and he'll be warm for a day;
  set a man on fire, and he'll be warm for the rest of his life."