Due to the usage of the document ready event

/* Initialise the date picker. */
$(document).ready(function() {
    $.datepicker = new Datepicker(); // singleton instance
    $(document.body).append($.datepicker._datepickerDiv).
        mousedown($.datepicker._checkExternalClick);
});

the datepicker becomes inflexible in its usage. Using a bottom script approach (instead of DOM ready) still requires the ready event to initialize a datepicker on the page to prevent an error ($.datepicker has no properties).

Quickfixed it by moving the datepicker initialization into the datepicker method itself, although I may have overlooked the impact of this:

$.fn.datepicker = function(options){

    /* Initialise the date picker if necessary. */
    if (!$.datepicker) {
        $.datepicker = new Datepicker(); // singleton instance
        $(document.body).append($.datepicker._datepickerDiv).
            mousedown($.datepicker._checkExternalClick);
    }

    var otherArgs = Array.prototype.slice.call(arguments, 1);
    ...
};