fieldset.js

$('fieldset.collapsible:not(.tao-processed) > legend > .fieldset-title').each(function() {
	var fieldset = $(this).parents('fieldset').eq(0);
	fieldset.addClass('tao-processed');
	// Expand any fieldsets containing errors.
	if ($('input.error, textarea.error, select.error', fieldset).size() > 0) {
		$(fieldset).removeClass('collapsed');
	}
	// Add a click handler for toggling fieldset state.
	$(this).click(function() {
		if (fieldset.is('.collapsed')) {
			$(fieldset).removeClass('collapsed').children('.fieldset-content').show();
		}
		else {
			$(fieldset).addClass('collapsed').children('.fieldset-content').hide();
		}
		return false;
	});
}); 

What the what now?

also, you can switch this $(this).click(function() { if else } with $(this).toggle(function() { }, function(){} ) which is way awesomer (because you can have as many as you want...)

This is actually copied from Drupal's collapsible fieldset functionality. It has to be run again on dynamically added fieldsets.

$('fieldset.collapsible').bind('click', function(){
    $(this).toggleClass('collapsed'); // where collapsed is display:none or height = 10px, overflow: hidden (or whatever hides the fieldset)
})