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It may make sense yet.  MTOM is a Candidate Recommendation - Omri breaks down the three specs.  This will be supported in WSE 3.0 and in Indigo, and appears to be the final and official way to do attachments with SOAP.  Only took four tries.

Omri explains why it took so many tries in this post from last year, and includes a series of examples that illustrate the DoInclude header and an inline encoded attachment as well as one outside the Envelope. 

He sums it up well (emphasis mine):

[The W3C's MTOM (which is based on PASwA) is] the attachments technology that we believe gives us an adequate interoperable solution for attaching large objects to SOAP messages, while still retaining the goodness of the XML Infoset, thereby enabling composition with other WS technologies such as WS-Security. [Omri Gazitt]