In the sandbox try the following code:

		<combobox autodrop="true" readonly="true">
                        <comboitem label="" />
			<comboitem label="Simple and Rich" />
			<comboitem label="Cool!" />
			<comboitem label="Thumbs Up!" />
		</combobox>

then select the combobox and try to navigate. If nothing is preselected (empty element) it will not be possible to navigate. If an element with label is preselected navigation is possible as long as you will not highlight the element with empty label.