This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
The problem isn't you but Tomcat5. It incorrectly handles the combination between form based authentication and post method. So when you correctly type in your user and password it authenticate correctly but then generates a GET request instead of the POST request. Because the doGet is not implemented you receive an error. if you implement the doGet() it should work fine. Or change the authentication mode (to BASIC) and restart Tomcat