I am using Eclipse Helios and Tomcat 7 and Ubuntu Lucid Lynx. When developing the web.xml, I started with the web.xml file supplied with Tomcat 7. I have been able to successfully run this web app independently from Eclipse Helios (directly from /var/lib/tomcat7/webapps/HearingImpaired). This example is taken directly from the Head First Servlets & JSPs textbook. I have been working on this problem for more than 2 hours. I don't see anything wrong with my web.xml file (Do you?).
HearingImpaired is the name of the Dynamic Web Project in Eclipse Helios.