I don't know how was the plugin the time the question was asked (2009), but I faced the same problem today and solved it this way:
Give your select tag a name attribute. For example in this case
<select name="myselect">
Instead of working with the attribute value="default" in the tag option, disable the default option as suggested by Jeremy Visser or set value=""
<option disabled="disabled">Choose...</option>
or
<option value="">Choose...</option>
Set the plugin validation rule
$( "#YOUR_FORM_ID" ).validate({
rules: {
myselect: { required: true }
}
});
or
<select name="myselect" class="required">
Obs: redsquare's solution works only if you have just one select in your form. If you want his solution to work with more than one select add a name attribute to your select.
Hope it helps! :)