Nonetheless, Sender-ID compliant sites MUST use DNS recursive servers
that support EDNS0 [RFC2671] and [RFC3226] in order to be able to
receive large DNS RR sets
I'm not sure this is exactly right. I assume that the intent is not
to _require_ applications to use recursive servers (eg an
implementation might incorporate its own EDNS0-compliant full-service
resolver).
I'm also rather uneasy about the notion of a 'sender-ID compliant
site'.
I think that the notion of a compliant site probably belongs in a BCP;
standards track RFCs define protocols, not standards that sites should
meet.
Unfortunately you need that (or a similar) notion to say what you want
to say; the Sender ID implementation doesn't have to support EDNS0
itself; the systems administrator at the site just has to point it an
an EDNS0-compliant recursive server.
I realize that it's a bit late to be suggesting that we need _another_
draft, but IMHO that's where such a requirement belongs.
FWIW, it could be a very short draft :-)
-roy