In the definition for the focusable function (jquery.ui.core), part of the check to test whether or not an element can receive focus is to check its visibility. Line 184 contains the comment, "the element and all of its ancestors must be visible". It would seem inconsistent that in the core there would be a test for visibility on the called element and all ancestors, but in datepicker the test for an element about to receive focus is simply "is this element visible". There doesn't seem to be a plausible reason why one function should have an ancestor check, but another does not. Especially since datepicker does check for some sort visibility before trying to call focus.
Additionally, if the code is not modified I would argue that the defect still stands as "valid" because the documentation is incorrect. Nowhere does the datepicker docs state that a form field using datepicker is required to be visible. In our use case, a different action other than focusing on the element to which datepicker was attached caused the datepicker to display. Additional confusion was caused by the fact that only IE manifested the problem. If calling "show" requires the element to be visible, though I'm doubtful since the code partially supports checking for visibility before setting focus, then it might be best to clearly state this.