W3C released a working draft of the SOAP Message Transmission Optimization Mechanism ...

The first part (Abstract Transmission Optimization Feature) describes an abstract feature for optimizing the transmission and/or wire format of a SOAP message by selectively re-encoding portions of the message, while still presenting an XML Infoset to the SOAP application.

This Abstract Transmission Optimization Feature is intended to be implemented by SOAP bindings, however nothing precludes implementation as a SOAP module.

The usage of the Abstract Transmission Optimization Feature is a hop-by-hop contract between a SOAP node and the next SOAP node in the SOAP message path, providing no normative convention for optimization of SOAP transmission through intermediaries. Additional specifications could in principle be written to provide for optimized multi-hop facilities provided herein, or in other ways that build on this specification (e.g. by providing for transparent passthrough of optimized messages).

The second part describes an Inclusion Mechanism implementing part of the Abstract Transmission Optimization Feature in a binding-independant way.

The third part (HTTP Transmission Optimization Feature) uses this Inclusion Mechanism for implementing the Abstract Transmission Optimization Feature for an HTTP binding.

Fact: actually the stuff is meant to standardize the sending of (binary) attachments as MIME Multipart and other standards (like it is done via e-mail already)