I've just seen this mod here:
http://wannabe.contropa.com/facebook-connect-for-phpbb2/217/
I'm assuming the other mods were for phpBB3, so maybe this will be a tad easier.
I'm gonna have a play with it myself. I'll reply if I have any luck
UPDATE:
I tried it on a vanilla install first... which was met with mixed success. I used my facebook account fine, and so did my friend, but the third person then logged in my facebook account.
There was some error when I tried deleting my facebook user through the
ACP, so this may be where I caused problems in the database.
I'll try again, and not delete any users this time...
Edited by
McManus, Wed 24 Nov, 2010 15:19: Played with mod on vanilla phpBB2 first. Update on progress