One of the big missing pieces in middleware standards is a wire format for messaging. David Chappell posted a good write up of this problem on his blog.
The industry has spent a lot of time working on interoperable wire formats for RPC. One could argue that SOAP is intended as such a format for messaging, since the SOAP spec includes support for the document-oriented style. Or that XML provides a standard data format that messaging systems can use.
Unfortunately, as Steve also points out, this is an area that has not been well addressed in the past.
Obviously with the AMQP announcement we hope this will change.