I didn't expect to make this symptom into a series...
There is an interesting MSMQ 4.0 problem highlighted in the following KB article:
936502 A message is not received when you use the HTTP protocol to send the message from a Message Queuing client to a Message Queuing 4.0-based serverhttp://support.microsoft.com/default.aspx?scid=kb;EN-US;936502
Basically, don't put ".SVC" in your queue names as otherwise IIS will get confused IF you have chosen WCF activation:
The handlers for Windows Communication Foundation (WCF) are looking for this file extension in the PUT request and will grab anything that looks like it is for them.
To get round this you need to ensure that the MSMQ handlers see the PUT request before the WCF handlers: