Unfortunately I can't change from JBoss AS 5.1 for now, so I tried some workarounds and now I can preview birt reports.
I Chaged to Jboss Tools 3.3M2 with Eclipse Indigo and followed the tips from Tomas Cerny in this post http://www.seamframework.org/Documentation/WhatHappensWhenYouDeploySeamAppInJBoss5#H-TheQuickFixes
The only change needed was to remove
<bean name="SeamMTMatcher" class="org.jboss.seam.integration.microcontainer.deployers.SeamTempModificationTypeMatcher"/>
from
$JBOSS_HOME/server/default/deployers/seam.deployer/META-INF/seam-deployers-jboss-beans.xml
I don't know the true impact of this change, but now I can test birt with JBoss Tools.