Turns out the answer is both easy and blindingly obvious once you think of it: the iframe onload event. And while we're add it, we'll add a onerror event too.

In my previous program, I had had a hidden iframe, and on every update, I would reset the source for the iframe using the location property.

We'll still do that, but add a new function:

       <iframe name="hidden" src="CometCount" frameborder="0" height="0" width="100%"
       onload="restartPoll()" onerror="restartPoll()" ></iframe>

Note the onload and onerror events - whenever the server closes the connection, these will be called. And here's the function that's called:

            var retries = 0;
            function restartPoll() {
                if (retries++ > 10) {
                    alert("The connection has errored out too many times");                    
                } else {
                    hidden.location = url;                    
                }
            }

Also, I've added a retry limit in there - it wouldn't do to have the client go into a fatal spin just because the server is down.

Now when the server closes the connection (from a timeout, or an error), the client will continue to function, automatically calling back into the server. Not a solution you'll want for every situation, but useful enough, especially for our small example.

Lastly, there was a bug in my previous version under IE - sorry about that. It turns out that if you send a POST via IE, you need to have a content body, or IE gets fussy. The fix is to change the line

            xhReq.send(null);
to
            xhReq.send("null");