[jQuery] DatePicker not working--how to debug?

[jQuery] DatePicker not working--how to debug?


I'm struggling to figure out why DatePicker is not working for me. I'm
including the following files (and they're all loading fine). Is there
yet another file required?
ui.core.css
ui.theme.css
ui.datepicker.css
jquery-1.3.1.min.js
jquery-ui-1.6rc5.min.js
jquery-ui-datepicker-1.6rc5.min.js
The form input looks like so:
<input type="text" name="foobar" id="foo" class="DatePicker" />
The following has no effect at all (including errors):
$(document).ready(function(){
    $(".DatePicker").datepicker();
});
If I change the above to this, the alert properly shows "foo".
$(document).ready(function(){
    $(".DatePicker").each(function(){alert(this.id);});
});
Can someone suggest some way to go about debugging this?