It's marked as "takes a lot of effort" and has no official releases, but this may be of use to you:

http://drupal.org/project/phpbb

I have no experience with it other than viewing that page just now. :-)

On Thursday 15 February 2007 2:05 pm, will hall wrote:

hi all, we have drupal and phpbb running side by side but as distinct creatures with their own databases and logins and usernames.

we would like to not use drupal forums but continue to use the phpbb forums. we do need however to have a single username and password for both.

so when someone registers for drupal, or registers for phpbb, there is a single username and password that will log them in to drupal and phpbb.

we don't necessarily need for them to be logged in to both with one login screen, we just need them to be able to use both with the same username and password.

any ideas on how to do this?

right now we have 2 usernames and 2 passwords for everyone who is using both the forums and the drupal site. it is confusing, especially to new members, and helping people sort this out now represents the biggest portion of our incoming admin@ mail.

viva drupal,

-- will