On Debian 6, this can also happen when you change the innodb_log_file_size parameter... some timesSometimes mysql does not start.. butstart; other times, it just starts up bud disables innodb, but the InnoDB engine.. is not enabled. soThe solution is to remove ib_logfiles from /var/lib/mysql and restart mysql.
And readSee Issuethis issue in drupal.org.