JBoss 4.0.3SP1 admin guide claims that

"When a WAR file is deployed, it is associated by default with the virtual host whose name matches the defaultHost attribute of the containing Engine."

This is false, and thus default WAR deployments are broken when there is no host named "localhost" – or even if there is such virtual host, but WARs should not be deployed there by default.

It seems that, regardless of what Hosts are configured, and what the defaultHost is, if WAR does not have a jboss-web descriptor that specifies the virtual host, the WAR always deploys to "localhost". If there is no Host with that name configured, the WAR is inaccessible.