cache directory not set to default on initial install - throw notice on status page if this happens

If it saves it directly in the /cache dir then the static file will not be served unless you rewrite the rules. For clearing the cache it also might be problematic. Did you set the Cache file path: on the performance page?

If those are set correctly, can you run this in a php block and tell me what it gives you?

echo variable_get('boost_file_path', boost_cache_directory(NULL, FALSE));

I should probably add a logic check to the boost.install file so it will throw an error on the status page if the boost_file_path variable isn't set correctly. I could have it throw other errors as well...

It's supposed to default to "cache/www.example.com" if i remember correctly; www.example.com being your site.

function boost_cache_directory($host = NULL, $absolute = TRUE) {
  global $base_url;
  $parts = parse_url($base_url);
  $host = !empty($host) ? $host : $parts['host'];

  // FIXME: correctly handle Drupal subdirectory installations.
  return implode('/', !$absolute ? array('cache', $host) : array(getcwd(), 'cache', $host));
}

NULL is seen as empty by php, so it should grab your host name from the $base_url variable.
http://www.php.net/empty
Did you follow step #7 in the install file? This won't matter since your clearing the cache dir with your own cron job.
http://drupal.org/node/367081#comment-1504894

Would you mind running this php code block for me?

echo boost_cache_directory();

Did you install & enable boost via cvs/command line?

I can't seem to find the reason why boost's boost_file_path variable got set to cache in your case. The code that initializes that is

define('BOOST_FILE_PATH',            variable_get('boost_file_path', boost_cache_directory(NULL, FALSE)));

http://api.drupal.org/api/function/variable_get
which then gets saved when you do a submit on the performance page because of this code

  $form['boost_file_path'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Cache file path'),
    '#default_value' => BOOST_FILE_PATH,
    '#size'          => 60,
    '#maxlength'     => 255,
    '#required'      => TRUE,
    '#description'   => t('A file system path where the static cache files will be stored. This directory has to exist and be writable by Drupal. The default setting is to store the files in a directory named %default-path under the Drupal installation directory. If you change this, you must also change the URL rewrite rules in your web server configuration (.htaccess for Apache, lighttpd.conf for Lighttpd), or caching will not work.', array('%default-path' => boost_cache_directory(NULL, FALSE))),
    '#weight'        => -5,
  );

I can't give you a good answer as to why this happened to you. My only guess has to do with the $base_url, but that would only occur if this module was enabled via the command line.

I think the easiest way to prevent this error in the future is to set that variable in the install file. Thus when the module is first installed, it gets set to boost_cache_directory(NULL, FALSE).

I need to throw an error in the status if it's set to cache/.

patch to check the cache dir

committed

After clearing cache, if cache is empty upon viewing status page, boost will complain. This fixes that.