I ran into this problem when I had to call a javascript on page load after my server-side code threw an exception to the user.
If you want to call a javascript function on page postback, you'll need to add an EventListner to the current document.
<script language="JavaScript" type="text/javascript">
function doSomething(){.... }
if (window.addEventListener)
window.addEventListener("load", doSomething, false);
else if (window.attachEvent)
window.attachEvent("onload", doSomething);
else if (document.getElementById)
window.onload= doSomething;
</script>