Role of AS2?

Can anyone comment on the relationship of Web services to AS2
(http://www.ietf.org/internet-drafts/draft-ietf-ediint-as2-13.txt).  Is
this another transport protocol (like SMTP) that can be used underneath
Web services?  Note that AS2 is built entirely on HTTP, so it doesn't
seem to be exactly the same kind of thing.  If it is, however, an
alternative for messaging WS's, does AS2 have some role that should be
recognized in the security and reliability aspects of Web services?

One of the reasons I am asking about this is that AS2 has recently
become a pretty hot topic in business because WalMart, the prototype of
the 800 lb gorilla, is in the process of mandating that if you want to
do business with them you have to use AS2.  This is, to put it mildly, a
very significant driver.  Whether or not AS2, as such, should be
incorporated somehow into the WSA, I would personally appreciate any
insights that you folks have about it.