Using datepicker version 3.4. In IE the jquery.datepicker object is undefined in the onload handler for a page we were building. What worked for us was to simply move the singleton instantiation

$.datepicker = new Datepicker()

out of the $(document).ready handler.

So the code now looks like this:

/* Initialise the date picker. */
$.datepicker = new Datepicker(); // singleton instance

$(document).ready(function() {

	$(document.body).append($.datepicker._datepickerDiv).
		mousedown($.datepicker._checkExternalClick);
});

This works perfectly for us.