Here is an example showing how to grow a progressbar smoothly. It might be nice to have on the progressbar demo page.
<script>
$("#progressbar").progressbar({value: 33});
// Use instead of progressbar("value", 67).
// Caveat: doesn't support ARIA, since _refreshValue not called.
$("#progressbar .ui-progressbar-value").animate(
{
width: "67%"
}, {queue: false});
</script>