Didn't discover a way to do this but I ended up using an acceptable work around of using the buttonImage feature with buttonImageOnly and then disabling the date field.
$("#id_date").datepicker({
dateFormat: 'yy/mm/dd',
showOn:"button",
buttonImage: "/icons/calendar.gif",
buttonImageOnly: true });
$('#id_date').attr("disabled", true);
If you are doing this on a form, make sure you re-enable the field before submitting or serializing the form or the value won't be sent (on iPad at least). I do the following
in my submit function:
$('#id_date').attr("disabled", false);
var dataString = $('#the_form').serialize();
$.ajax({
type: "POST",
url: 'myurl',
data: dataString,
...