If you're trying to create an accessible form, "the value of the for attribute must be the same as the value of the id attribute of the form control" [1]. Since the ComboBox widget generates a new element and hides the original select element, the for attribute no longer matches this new input element. I think it would be good if the ComboBox widget would somehow keep the for attribute of the associated label element in sync with the newly generated input element.

[1]: http://www.w3.org/TR/2012/NOTE-WCAG20-TECHS-20120103/H44