You could check if the input field is empty or not, if its not, don't auto update. Then when they post you would empty the input and your auto updating would continue.
$(document).ready(function() {
$.ajaxSetup({ cache: false });
autoUpdate('URL_TO_FEED_AS_HTML', '.discussion');
setInterval(function() {
if ($('input').val().length != 0) {
autoUpdate('URL_TO_FEED_AS_HTML', '.discussion');
}
}, 10000);
});
As recommended by Rory McCrossan, You would get much better performance and functionality from appending the latest discussions. You could do this by querying anything newer than the current timestamp (timestamp of when they last queried and not just before you query results), then use append() to add the data onto the page.