With reference to http://drupal.org/node/561990#comment-6861664 (see comments #39 to #41), I understand that the code for lock_acquire actually suppresses this error, but Boost does not ignore the following error...

I see it in the /admin/reports/dblog under "boost" type:

There are php errors on this page, preventing boost from caching. ERROR:
Array
      [type] => 512
      [message] => Duplicate entry 'variable_cache_regenerate' for key 1
query: INSERT INTO semaphore (name, value, expire) VALUES ('variable_cache_regenerate', '151849102550d2f24540f987.74939510', 1356001891.2661)
      [file] => /var/www/mysite/includes/database.mysql.inc
      [line] => 135

Using: Drupal 6, Boost, NGINX, InnoDB tables (most of them)