According to the Java Look and Feel Design guidelines, Return/Enter is supposed to close a combobox list and commit any change. If the list is already closed, it would be nice if Return/Enter would active the default button on the dialog box.

This makes the most sense for a non-editable combo box. Consideration should also be given for whether this makes sense for an editable combo box (maybe the first return commits any edit, and the second one is passed to the default button/container?)