mercury pantheon varnish pressflow memcached

We just moved our test site over to an AWS hosted solution based on the 1.0 Mercury AMI (love it!) with plans to take it live in the next couple of weeks. We are using a custom shopping cart solution that allows anonymous visitors to 'add' items to their cart via a URL (not via form submit). When the link is accessed, the custom module will instantiate a session (which in turns sets a cookie) to ensure that varnish is no longer serving cached pages.

This is a question that had been asked before and discussed elsewhere. For instance, https://wiki.fourkitchens.com/display/PF/Modules+that+break+caching%2C+a... provides a list of modules that break the external and aggressive caching - some of them does not play well with normal cache either.

I need Drupal for Facebook and Organic Groups, both of which are listed as incompatible when external cache setting in the performance page is enabled.