I've been searching for a while, but the only Akismet modification I have managed to find only checks
whether it's spam, and does not allow the user to submit
spam to the database. This, in my mind, is lazy and wrong, so I'm going to make a proper Akismet modification which, when a post is deleted as spam, submits the post to Akismet.
It won't just delete a comment if it detects that it is spam. Although that will be one of the option, there will also be the option to put them in the moderation queue (and report to akismet if it isn't spam), or to present them with a captcha. If they fail the captcha, the post be deleted, but if they succeed their post will either be put in the moderation queue or posted (up to the admin).
It will be based on the following link and the class linked to from that link: http://randombyte.net/wiki/phpbb_akismet
Please give me your thoughts, I will start development later today