I think you're perfectly reasonable to expect to be able to do this, after all, when clients are connected for comet or websockets, it's likely that each one will make less than one request per second (depending on what you are using the channel for of course). This means that you aren't looking for a framework that can handle 10,000 requests per second, you are looking for a framework that can handle 10,000 requests at the same time. This distinction is often missed.
Most frameworks operate under the one-thread-per-request model, where in order to complete a request the thread will perform blocking requests to other resources and this simply won't float in your case. With each thread given it's own stack (that on many OSs is a fixed size, often 8MB), it's not viable to have 10,000 threads running simultaneously.
Another model is to have a small pool of threads that are used to handle events via callbacks. And it's this event-driven model that I'd recommend for your project. Recently, node.js has become the major player in event-driven web application design. You should also note that the server software that you use should also be event-driven, so lighttpd, nginx, cherokee are good matches there.