The ARIA 1.1 spec for combobox needs to clearly specify whether to put the label on the parent (combobox) element, or the child (textbox) element.

Example 5 does show it on the combobox element and not on the textbox, but are examples normative?

An argument for having the label on the combobox element is that AT need to know the label of the combobox in order to list them by name in a form fields dialog, otherwise you have problems like this "unlabelled comboboxes" JAWS issue.

An argument for having the label on the textbox element is that it takes focus, so AT will want to speak the name.

I don't think it makes sense to ask authors to put the label on both.

Unfortunately, neither JAWS, NVDA, nor VoiceOver seem to fully support ARIA 1.1 comboboxes yet.
Focus goes to the textbox, and the screen readers just say "edit, type in text". The user won't even know they are in a combobox. (If the textbox has a value, that is read as well, but still no indication of combobox).