If using (only) Java runtime packages ("openjdk-7-jre" or "openjdk-7-jre-headless"), OpenNMS 1.11.90 and 11.11.91-0.20130422.3 (the current snapshot) will not display the web login page when going to /opennms.
daemon/uncategorized.log seems to have the reason:
2013-04-22 23:07:59,472 WARN [qtp1991606348-386] /opennms: Something went horribly awry.
This message at the jetty-users mailing list says "The issue is that we updated the version of jasper (jsp engine from Glassfish) that we use in 7.5.0 and it insists on using the in-jvm compiler if you're using jdk1.6 or above. Therefore, you need to have a full jdk installed (until we can get a patch into a release, stay tuned for that)."
This appears to be the case with OpenNMS 1.11.9x. After installing a Java compiler package ("openjdk-7-jdk"), the login screen appeared.