If you're not fussed about Internet Explorer 7 or less, then you can do this in CSS, which is where it should be ideally (as it is presentation, not content):
div.Syb> div:before
{
content: '+';
}
div.Syb> div.hidden:before
{
content: '-';
}
Or you could play around with background images.
Then just simply use toggleClass() to give your div a .hidden class.
If you want to do it in straight up jQuery, you'll probably have to change your markup a little:
<div class="Syb">
<h4><span>+</span>Title One</h4>
<div id="one">Text</div>
<h4><span>+</span>Title Two</h4>
<div id="two">Text</div>
<h4><span>+</span>Title Three</h4>
<div id="three">Text</div>
</div>
Then you can simply change your jQuery as such:
<script type='text/javascript'>
$(document).ready(function() {
$('div.Syb> div').hide();
$('div.Syb> h4').click(function() {
var span = $(this).children('span:first');
span.text(span.text()=='+'?'-':'+');
$(this).next('div').slideToggle('fast')
.siblings('div:visible').slideUp('fast');
});
});
</script>