// the function

function set_value_toggle(t) {
	default_value = t.value;
	
	t.addEvent('focus', function() { 
		if (t.value == default_value) t.value = '';
	});
	
	t.addEvent('blur', function() { 
		if (t.value == '') t.value = default_value;
	});

}

// the call

window.addEvent('domready', function() {
	set_value_toggle($('desired_username'));
});

// the form

<input class="textfield" name="desired_username" id="desired_username" type="text" tabindex="4" value="desired username" />