Fellow Liiper Lukas writes about a “ Reverse proxy cache invalidation service” for Varnish and Symfony2. We use that for a big news site, where caching and “on time” delivery of articles is of utmost importance. The service is not done yet, but if all goes well, it will be done with some of Liip's innovation budget and consequently open sourced later.
Now go and read the whole thing, it's a very interesting approach in my opinion.