The problem seems to be that recent Tomcat versions still package a few Java 1.4 compatibility .jars in /usr/share/tomcat5.5/common/endorsed. This prevents Tomcat from loading XPathFactory.
(see http://www.openrdf.org/forum/mvnforum/viewthread?thread=1328). Removing the two .jars fixes the problem for Solr, but it may cause problems for other applications or other VMs. Jan-Pascal
