How to remove Page cached by boost message in view source
This is not possible in the 7.x-1.x version of the module (unless you patch the code). You should file a feature request in the Boost issue queue.

If you want to temporarily patch the code, open the file boost.module, around line 1145:

  $types['text/html'] = array(
    'title'           =&gt; t('HTML'),
    'description'     =&gt; t('HTML output, usually a webpage'),
    'extension'       =&gt; 'html',
    'enabled'         =&gt; TRUE,
    'gzip'            =&gt; TRUE,
    'lifetime_max'    =&gt; 3600,
    'lifetime_min'    =&gt; 0,
    'comment_start'  =&gt; NULL, // &lt;--- set to null
    'comment_end'    =&gt; NULL, // &lt;--- this too
    'variants'        =&gt; array(),
  );


If you would like to contribute a patch, which would be greatly appreciated, the right way to do this would be to modify the boost_exit() function, around line 315:

  // Add note to bottom of content if possible.
  if ($_boost['matched_header_info']['comment_start'] &amp;&amp; $_boost['matched_header_info']['comment_end'] &amp;&amp; $_boost['matched_header_info']['comment_footer_enabled']) {
    $expire = $_boost['matched_header_info']['lifetime_max'];
    $cached_at = date('Y-m-d H:i:s', REQUEST_TIME);
    $expires_at = date('Y-m-d H:i:s', REQUEST_TIME + $expire);
    $note = "\n" . $_boost['matched_header_info']['comment_start'] . 'Page cached by Boost @ ' . $cached_at . ', expires @ ' . $expires_at . ', lifetime ' . format_interval($expire) . $_boost['matched_header_info']['comment_end'];
    $data .= $note;
  }


You would also have to set the default (TRUE) in boost_boost_storage_types(), and to the $keys in boost_get_storage_types() so that it does the variable_get(). Bonus points for also exposing it to the admin interface.