Boost not caching anything. is_cacheable always set to false
Looking at drupal_page_is_cacheable, it appears that there are 4 functions that use the cacheable flag.  drupal_page_get_cache and drupal_page_set_cache are to be expected, and use it simply as a flag.  Unfortunatley, drupal_session_initialze and drupal_set_message set cacheable to false.

In session initialize, if a user has visited your site at all and has a session cookie and any session variables are set then the page will not be cached.  Similarly, if you set any messages during the page generation, it will also not be cached.  While the former may have benefit, the later (IMO) is completely ridiculous.

Also, this issue on Drupal.org, but that really only addresses the first issue of session variables and cookies.

... commenting out the 'return' fixed the issue for me.  404 and 403 still not cached and you can specify other pages to be excluded in the boost settings page.