in IE6,

/* Generate the date picker content. */
_updateDatepicker: function(inst) {
    inst._datepickerDiv.empty().append(inst._generateDatepicker());
    var numMonths = inst._getNumberOfMonths();
    if (numMonths[0] != 1 || numMonths[1] != 1) {
	inst._datepickerDiv.addClass('datepicker_multi');
    } 
    else {
	inst._datepickerDiv.removeClass('datepicker_multi');
    }
    if (inst._get('isRTL')) {
	inst._datepickerDiv.addClass('datepicker_rtl');
    }
    else {
        inst._datepickerDiv.removeClass('datepicker_rtl');
    }
    if (inst._input && inst._input[0].type != 'hidden') {
	inst._input[0].focus();
    }
},

will reported "undefined" in "inst._input[0].focus();"

when I use "$('#' + inst._input[0].id).focus();" instead of "inst._input[0].focus();", it will be ok.

I guess in IE6 it is not support "focus()".