The JAX-RS features of CXF are currently not properly integrated in JBossWS-CXF. Some work still need to happen and we actually still need to decide what solution to use (RESTEasy vs CXF JAX-RS impl) when installing JBossWS-CXF.
I suggest you monitor https://jira.jboss.org/jira/browse/JBWS-2670 which is about the same topic, generally speaking JBWS 3.3.x should address this JAX-RS topic.
Salve Alessio, come va ?
We have been playing around with JAX-RS and trying to make it work in JBoss for two days (and we got almost nowhere - and keep getting all sorts of exceptions: http://www.jboss.org/index.html?module=bb&op=viewtopic&t=163683 amongst other errors).
I think something like your post should go in a very prominent location in the documentation (there is almost nothing mentioned on the JBoss site - or did we miss it ?). As energetic developers, we found the version of the Apache CXF included in the jbossws stack and tried to follow the Apache CXF documentation reasoning that anything supported by APpache CXF will be available through JBoss - and hence JAX RS 1.0. Maybe we were over-zealous.
Alessio, what kind of timelines are you looking at for JBossWS 3.3.x ? I am personally interested in this. Is there anything I can do to help you out ? Some particular piece of engineering which needs to be done ? Are you short on resources ?
We plan to have the first release of 3.3.x in Q1 2010, I'd say not before March 2010 (http://www.jboss.org/community/wiki/JBossWSRoadmap) and yes, if you're interested in this, your help would probably be interesting and accepted. Perhaps you could start looking at the integration project structure  while we start thinking about the way to go (RestEasy or not) next week or so. Then we might think about a way for you to start contributing, sounds good?
I'll send you an email during the weekend...