You have to put jndi.properties in your classpath.
This message is a bit cryptic :-)
"or in an application resource file: java.naming.factory.initial"
I had the same problem, actually with 2.4.3 it worked fine , in 2.4.4 suddendly I got this error.
But even when I now include the jndi.properties in the classpath (I have a standalone test client, running via JUnit) it doesn't work. I still get that exception.
Any suggestions why the jndi.properties is not read? When using the client via a servlet running with Tomcat 3.2.3 then everything works. Just problems with the standalone client for testing.