This is the package to worry about
www.v-eva.com/products/Blog-Comment-Poster.html
no url tags as I don't want to advertise it yet more on google.
I've read on blogs that among other spam it advertises itself to show to site owners how it can get past captchas etc.
It claims to defeat askimet which is considered one of the best anti spam plugins - its free and worth checking out.
Not had chance to check these out (don't know anything about php/bb anyway) but
this is the official antispam mods page for PHPBB
http://www.phpbb.com/customise/db/modifications/anti+spam-9 with ratings out of 5.and there's quite a few.
http://recaptcha.net/whyrecaptcha.html I know this one is popular
http://www.thefreecountry.com/php/captcha.shtml list of php captchas with descriptions - not sure how old it is
Hope these links are of some use.