Hey,
between rc1 and rc2 the ui-autocomplete class was moved from the input element which triggers the suggestions to the menu showing them. While i don't care if the menu has the class .ui-autocomplete-menu or .ui-autocomplete i think there should still be a class given to the input element identifying it as a autocomplete widget.
If you think the .ui-autocomplete is unsuitable for the input, you could still add the class .hasAutocomplete (like .hasDatepicker) or something like that.

I ran into that problem because i assign combobox buttons to some inputs, where i want to check if they have autocomplete enabled.

Hope you'll consider to re-add some identifying class to the input.

Thanks