It's clearly my fault. I followed this instructions https://community.jboss.org/wiki/JBossAS7EnablingJASPIAuthenticationForWebApplications and it works like a charm. It seems that the subject is being restored somehow by the WebJASPIAuthenticator valve, but it is not warking with my ServerAuthModule.
This is my ServerAuthModule code, base on Jaspic module for CAS found here http://code.google.com/p/cas-jaspic/
Removed useless code and jar file