I've tried several times to create a new project using the GWT Designer wizard and then I've configured the project for using GWT-Ext. I have always the error. I'm using Eclipse Galileo Java EE IDE for Web Developers - Build id: 20090621-0832 and gwt-windows-1.6.4.
After the unsucessful tries, I decided to configure the proyect by myself. I created the GWT project using de wizard but I added "manually" the required configuration for GWT-Ext. The result was that the project worked perfectly. I took the GWT-Ext configuration files and the jar gwtest.jar file from the GWTExt_test you left on this forum. Then, I've realized that the gwtext.jar file added by the wizard is different from the one present in the test project. As soon as I replaced the former jar file with the latter one, the error disappears. I've tried to resolve which versions have the different jar files, but I couldn't.
So, I hope you can find the problem.