By default, this will strip the Set-Cookie response header before storing a cacheable response.
Gemfile.lock was correctly in the .gitignore file, but was also committed to the repository. See http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/ Furthermore, it was locked to a version of memcached that I could get to compile on Lion.
Prior to this change, rack-cache would always strip any If-None-Match and/or If-Modified-Since headers from the request before passing along to the backend in an attempt to retrieve a response to put in the cache (304 responses are not currently cacheable). This approach falls down when the response includes a 'Cache-Control: private' header (Rails default) because the response cannot be cached but will also never allow for client initiated validation. The downside to this change is that the cache will fill in more slowly when clients make conditional requests for public resources; it won't fill in until a non-conditional request is received. The upside is that validation works for non-cacheable private responses.