I would also imagine that some proxies let connections time out and
forcibly close the connection if the entire "page" hasn't been
recieved in a specified time. Since a DC implementation would keep the
connections open indefinitely, that is a problem.

I am not sure about this. I have seen some interactive forums running with simple HTTP and having bi-directionnal communication over a single HTTP connection without timeout on it (at least during the 12 hours I am watching it).

Eric