Before I answer the question, I'll just say that I'm almost convinced to write another Combobox implementation. Over the time this has been available, it's had more points raised about it than any other - so I now think the best idea is to code another one which does what everybody thinks it should
The thing with the current implementation is that the 'selected' item is only valid while the drop-list is visible. I used an implementation whereby I gave significance to the contents of the editbox potion of the widget above all else, over the months I have come to realise that this is not what most people expected.
setItemSelectState is a proxy method to the underlying listbox method with the same name. It's purpose is to allow you to specify the select state of a list item (which for Combobox means nothing if the list is not presently shown). It's use is recommended over setting the state on the ListboxItem directly, because it respects the selection options set on the Listbox (like Multi-select and what have you).
Anyway, sorry for the confusion caused by my funky Combobox. If I get a couple of days spare some time, I'll write another one which does things a bit differently
CE.