First of all I'm a bit confused as to what is really included with Pressflow. Is Pressflow just a collection of modules or are there some deeper core hacks that aren't in D7? Would it be a reasonable solution to keep on going with our D7 install and adding the appropriate caching modules?
I understand that Drupal 7 works with Varnish out of the box, but then it also benefits from the Varnish module in some way. I'm curious about whether there's a killer-combo that people have generally converged on.