According to Bruce Johnson, they are going to add a setDebugId() method to Widget. You can set the id now using GWT, and we’ve used that ourselves to work with Selenium in our GWT apps:
DOM.setElementProperty(uiObject.getElement(), "id", id);
The difference here is that setDebugId() will set the id of the widget and all of its children in a deterministic way that allows you to easily identify parts of the widget’s DOM tree. For production, the setDebugId() will result in no code. Nice.
