I had been spiking around with RESTEasy in order to use some of the JAX-RS stuff for dealing with remote endpoints. We had a client library that already wrapped some of these end points and some DTOs that mapped to the XML the remote end point wanted. We wanted to use the JAX-B annotations to deal with the automatic conversions but kept getting the dreaded… 

java.lang.RuntimeException: could not find writer for content-type application/xml type: [.... Dto]

The secret to getting this to work was to ensure that you have the correct runtime libraries in the classpath, or more specifically the following jar.

GroupID: org.jboss.resteasy
ArtifactID: resteasy-jaxb-provider

Thanks to this thread for the help on that.