Yup
I bought eSase's phpbb3 mod. Initially it worked a treat, imported all users into phpbb3 from Dolphin effectively. New registrations were fine, showed up in phpbb3. No common avatars though, member profiles could not be imported into phpbb3, may be due to big differences between the two scripts, I'm no expert at this. One log in for both Dolphin and phpBB3 worked fine. I had problems when deleting accounts, this had to be manually done in phpbb3 as it was not syncronising with Dolphin in that regard. I also had to disable new registrations in phpbb3 as these were not being updated in Dolphin. So it was a one way registration process, but a two way log in. He may have update dthis since then, you may want to ask esase. ( http://www.expertzzz.com/Downloadz/view/2448 )
I had problems with the RSS feed to Dolphin and post/topic count in Dolphin's admin block. They would not progress from the point of integration.
It worked ok, I eventually did away with it and went with Orca (in retrospect this was a mistake as phpBB3 is a lot easier to use especially for the pre-ipod generation who make up most of my site's membership).
So now I am waiting for the new Dolphin version to be released hoping that Orca will be more pre-ipod user user friendly, if not, I will be re-intergrating phpbb3.
I did find esase a little reluctant to back up his mod but in all fairness
this may have been down to a language barrier. Others may have been
happy with his after sales support, I can only speak of my own
experience.