Position 1. In favor of XML:
Caller-ID (XML) has not had a huge publication rate. This *may* be a
case of domain owners vetoing XML, but there are many other
possibilities.
I do not think this has anything to do with XML vs SPF syntax; I think
it's a reflection of the level and kind of information you have to
gather in order to publish. As I have mentioned elsewhere, for most
domains publishing to catch all P.R.D. is much different exercise than
publishing the valid MAIL-FROM ip addresses. It's also much more
valuable information for the receiver, and required for tracking
reputation and accreditation by authoring domain rather than by
channel.
Margaret.