My site was hacked and code was injected into Akismet. So I deleted it and reinstalled a fresh copy (version 2.5.5) and cleaned up the other hacked files. I also installed a plugin called Better Security which blocked a login attempt already. It sent me an email about changed files. On the list was wp-content/plugins/akismet/error_log.
When I looked at the log, it has many copies of the same error:
PHP Fatal error: Call to undefined function plugin_dir_url() in /home/****/public_html/weblog/wp-content/plugins/akismet/akismet.php on line 32
So I guess Akismet isn't blocking the spam if it's dying.