JBoss 5.0 will have pluggable XACML Authorization support. We are integrating Sun's open source XACML implementation, that is an effort of Seth Proctor and Anne Anderson (both are XACML gurus).
Given this the following post from Burton's Identity Group is pretty interesting.
Waiting on XACML: An interop challenge for the industry
XACML is cool. Pain points are horrid xml, performance and lack of extensions for things like the EJB layer (where custom schema kicks in). Out goes the question of interoperability across JEE vendors.