I recently suffered no end of problems trying to get a JSF application to deploy correctly through Eclipse. I, of course, blamed Eclipse because lets face it it's the most flakey piece of software around. I was amazed then to discover that this was infact not the fault of Eclipse! The error message you get is as follows:

java.lang.ClassNotFoundException: 
com.sun.faces.config.ConfigureListener

This error message is issued at start up, is very persistant and stops your web application from deploying. The solution is to stop tomcat and delete everything under the work directory. I presume that items in the work directory are asking for this object.

This problem typically occurs when you try and upgrade to the latest version of JSF (which doesn't include this listener to the best of my knowledge) or when you try and swtich to MyFaces.

The work directory is either under the Tomcat root directory or, if you are working in Eclipse in the [workspace]/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/work directory.