ptth is an idea I have planning on implementing for a few years now. The basic idea is that you take normal HTTP semantics and reverse them, meaning that the client (from the TCP perspective) acts like a server (from the application perspective), and the server (from the TCP perspective) acts like a client (from the application perspective) and makes requests on the client whenever it feels like it. This is distinguished from most normal COMET semantics in that ptth retains all of http's characteristics even though the underlying transport is radically different looking at the TCP level.
When I was at Linden Lab, I advocated using this technique in the Second Life Viewer as a refinement of the Plain Old COMET implementation currently in use (which I also helped implement). I wrote a wiki page describing how the http Upgrade: header can be used to initiate a ptth connection, effectively turning a socket that the client opened to the server around, allowing the server to make requests on the client as if the server had opened a connection to the client (even though it didn't). I even did an implementation in Python showing how once the Upgrade: has been performed the semantics are exactly the same as normal http. This means with a little hackery it's possible (and in the Python case, almost trivial) to reuse existing http client and server libraries. All you have to mess around with is the setup of the socket; once both sides have an open socket and have agreed to Upgrade:, you just grab the underlying socket and pass it to the client or server library and away you go.
Even though I didn't get the chance to implement and deploy this technique in the Second Life Viewer and Server before I left Linden for Mochi, I still hope this gets implemented someday, as I think it is a very elegant and efficient technique. While implementing the real ptth Upgrade: in C++ will be more challenging than doing a quick Python prototype, once the dirty business of extracting sockets and injecting them into the client and server libraries used is complete, it should be a very reliable technique since at that point everything is exactly the same as normal http.
On Saturday the 6th we had a Mochi Hack Day at our office, and I was hacking on my perpetual hacking project, Pavel. If you don't know me personally and haven't heard me talk about Pavel, someday I'll flesh out the ideas behind it more fully in a series of web pages, but for now you can read this old blog post to get a rough idea of what it is. The post uses the term "graphical multiuser networked programming environment" to describe the basic idea. From the very beginning I conceived ptth as a vehicle for driving updates of the user interface to Pavel, so I decided to get down to it and actually implement it. Since I have implemented so many COMET servers at this point that I have lost count, it turned out to be almost trivially easy, and I had something working in a few hours.