Yeah, in 2.3 validation is a little bit broken, cause it was fully refactored after GWT 2.2. The things are working well in GWT 2.4-rc1 I think.
DynaTableRf contains a nice validation example. To check whether JSR-303 is working, just check that demo project but don't forget to remove this validation check from PersonEditorWorkflow.java
// Check for errors
if (editorDriver.hasErrors()) {
dialog.setText("Errors detected locally");
// return;
}