Looks like https://issues.jboss.org/browse/JBIDE-7932 https://issues.jboss.org/browse/JBAS-6843
Try to remove "Class-Path: jboss-seam.jar" from <ejb_project>/JavaSource/META-INF/MANIFEST.MF
It seems to be a bug of JBoss AS 6.0 (works fine in 5.*)
Also see Workaround Description in https://issues.jboss.org/browse/JBIDE-7932 to get rid of this error and not to break the project class path for Eclipse.