> Practically speaking, you typically run pgbouncer on the same box as postgres.

I've done this, long-term, in production. This is not how PGbouncer is designed to be used, and it causes serious problems under load. (I am still doing this, but I have it on my todo list to remove this glitch from my architecture, as it caused my entire site infrastructure to fail once during a launch.) Put simply: PGbouncer is single-threaded; it is not difficult to end up in situations where the one core PGbouncer is running on to be at 100% while the remaining cores (in my case, seven) running PostgreSQL are having no issue with the traffic.