Can I run Geomajas 1.4 with Apache Tomcat 6 on Ubuntu?

This should be fixed in 1.4.1 and higher versions of Geomajas. The catalina.out reports this:
INFO: validateJarFile(/usr/local/tomcat/webapps/majas-tutorial-1.4.0/WEB-INF/lib/servlet-api-2.3.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class

It is a warning, not an error. Your application will run fine as it is. In the next version the jar will be removed.

See also the following issue:
http://jira.geomajas.org/jira/browse/MAJ-369