In evolution's (or banshee and others) search entry there's a cool behavior that is a text displayed when the widget don't have the focus. The text is grayed and when you click on the entry the text disappears, letting you to type. Once the focus goes to something else, the text if back (unless the entry has some real content).

As many projects implement this "property" by hand, I think it will be valuable to have this in gtk+.