I get the same error with CXF 2.2.9 and JBoss 6.0.0.M4.
The problem occurred with the following directory structure:
test.war
WEB-INF
lib
cxf-2.2.9.jar
So just adding the CXF library to an empty war is enough to reproduce the problem.
I am afraid that class isolation solution may affect any other annotations that we may need JBoss to recognize in the war directory. I think this is a bug that should be fixed in JBoss.