In order to make the Authcache module compatible with the Logintoboggan module, which provides a block that shows the username of the currently logged-in user, I had to change the scope of the $user_name & $user_link variables, which were previously generated by authcache_preprocess_page, to be generated in authcache_preprocess.
I think this would be a helpful change, especially since the documentation doesn't make it immediately obvious that these variables are *only* available in page.tpl.php. It says "template files (e.g. page.tpl.php)", which suggested to me that page.tpl.php was only an example.
Anyway, I have attached a page which makes this change.