Sorry for the mistake: I'm actually testing on JBoss AS 4.2.2...
Looking at the JBoss log at startup I noticed the following lines:
... 15:13:55,934 INFO [ServletContextListener] Welcome to Seam 2.1.1.GA 15:13:56,059 INFO [Scanner] skipping class org/jboss/seam/resteasy/ResteasyDispatcher.class because it cannot be loaded (may reference a type which is not available on the classpath) ...
Do I need to add resteasy jars to the project?
As described in section 24.4.1 in the doc
get the RESTEasy libraries and the jaxrs-api.jar, deploy them with the other libraries of your application.
I was missing resteasy-jaxrs.jar...