The bad news is that this is not defined in the spec.
The good news is that because I could not get JSR154 to agree
to how welcome files should be handled, we have recently implemented
a change so that we support the same behaviour as tomcat.
The problem is that we use RequestDispatcher.forward to access
the welcome file, while the latest versoions of tomcat uses a redirect.
Earlier versions use something like a forward, but that does not match
the spec either.
In the 2.3 spec, filters do not apply to request dispatcher filters.
But since Jetty-4.1.0RC6, which has gone into JBoss 3.2.0b
you can set the redirectWelcome initParam on the default servlet to
achieve the same behaviour.