a strange behavior can occur when using WebSphere AppServer, WebSphere MQ and Spring Integration. additional headers you add via Spring Integration header enrichment can 'fail' to appear once it's gone over the JMS wire via MQ. this doesn't occur when using WebSphere MQ directly, only from the WebSphere container.
so, the solution appears to be in the queue configuration in WebSphere AppServer, the default setting means that RFH2 Headers are not appended to the message. though i'm not 100% on why this works, but when setting it to true, Spring Integration headers now work.
to find this in WebSphere V8 - go to the following
Resources > JMS > Queues > [your queue] > Advanced Properties
and check the box next to
"Append RFH version 2 headers to messages sent to this destination"
it works - not exactly sure how...