I've been looking forward to a phpBB MOD tying in Akismet's protection for some time.
And while it's great seemingly spammy posts are blocked, I think it would be much better to store them in a moderation queue of sorts, perhaps even listening them in the regular forums but visible only to admins and/or mods. The user making the post could be notified that their post is awaiting moderation.
This gives allowed users the chance to flag messages as spam which Akismet misses as well as flagging messages as "ham" that Akismet mistakes for spam. Akismet is notified each time a message is mis-flagged, and it is this ever-growing, ever-training aspect which makes Akismet so powerful.
Merely blocking the post up front does not provide the opportunity for Akismet to learn and adapt to what constitutes spam or ham.
At the very least, if spam gets through, there should be capability to flag it as spam with Akismet while deleting it from your board.