Validate select box

Just add a class of required to the select

<select id="select" class="required">

For starters, you can "disable" the option from being selected accidentally by users:

<option value="" disabled="disabled">Choose an option</option>

Then, inside your JavaScript event (doesn't matter whether it is jQuery or JavaScript), for your form to validate whether it is set, do:

select = document.getElementById('select'); // or in jQuery use: select = this;
if (select.value) {
  // value is set to a valid option, so submit form
  return true;
}
return false;

Or something to that effect.

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:

1. Give your select tag a name attribute. For example in this case

   <select name="myselect">

2. 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>

3. 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! :)