the zIndex for the autocomplete result list which is popping up is calculated with "this.element.zIndex() + 1"
the problem is that when the input element is not 'absolute' || 'relative' || 'fixed' the zIndex of the input is ignored but very useful for the Menu which is popping up.
In my example its an jquery autocomplete above an openlayers map - and the map was always over the auto complete list - i had no chance to get it above without tweaking jquery.ui.autocomplete.js ( its an dirty hack so i won't publish it as a patch here )
I'm not sure about how to do a real Patch which suits everyone. Perhaps passing an zIndex with the autocomplete Options is an option?