Ok, I added web.xml entry that exception message suggests to add:
<context-param> <param-name>com.sun.faces.expressionFactory</param-name> <param-value>org.jboss.el.ExpressionFactoryImpl</param-value> </context-param>
Now I'm explicitly pointing that I want to user jboss EL implementation of expression factory. However this entry seems to be ignored and the exception remains :(.
Allright, it seems that I finally solved this problem. The solution was to put archive jboss-el.jar from Seam distribution into commons/lib directory of Tomcat 5.5 or into lib directory of Tomcat 6 (and not in application's lib!). There is no need to add anything to web.xml.