We write pages to disk as compressed HTML, and make use of nginx's gzip_static and gunzip modules to serve them. So every time a person posts, we regenerate the applicable reply and index HTML. The high postrate boards are rebuilt with a daemon on a timer, since past a certain point (~1 post per second) it's wasteful to regenerate on demand given how long the script takes to run.

We essentially think of flat files on disk as a cache for the database, and don't employ a proxy cache or any other common HTTP proxies. It's a little unorthodox, but it works well for us.