With both the applet and the javascript. Both end up making a perpetual socket to the back end.
The applet, however, uses more CPU during updates. Mainly (we think) because it starts up a new javascript interpreter for each call.
The memory buildup can be solved exactly as you describe, but we'll probably refresh the page every 15 or 30 minutes, not every 30 seconds.
The current discussion, however, is whether or not the throbber will sow confusion amongst the ranks of the users. :-P