Also note that a proxy that uses TCP must be stateful as well, whether it forks or not. This has to do with reliability issues.

Why do you want state in a proxy? Certain services (like forking) simply require it. A sequential search proxy requires state; sequential search is the heart of services like follow-me and personal mobility. It's at the discretion of the implementor whether to use a stateful or stateless proxy. You can even be "super stateful", and use the Record-Route header to allow a proxy to be on the signaling path of all subsequent exchanges. This allows a stateful proxy to maintain call state in addition to transaction state.