> (not label or item or index)
I suspect 'item' is what you need actually. Specifically the 'selectedItem' property which returns the whole of the selected object from your dataprovider. From that you can then extract the selected object's properties.
e.g. selectedValue = myComboBox.selectedItem.value;
The example in tour-de-flex shows this - note: 'data' in that example is i think what you call 'value'.
http://tourdeflex.adobe.com/flex4.0/ComboBox/sample.html (right-click to view source)