I've implemented a pretty basic PHPBB single-signon plugin for b2evolution.
Basically, the way it works is when you go to a b2evo page, it looks for the phpbb cookie and if it's there, the cookie contains the user_id of the user you're logged in to phpbb as. So, using the cookie, it looks up your username in the phpbb database, and then it tries to log you on to the b2evo database using the same username. If no user exists in the b2evo database with that username, it creates a new one.
There's probably some tweaking that needs to be done, but so far it seems to be working fairly nicely.
I'm not, at this stage, planning to release my plugin, since it's a bit rough around the edges, but if there's enough demand, I might spend some time to clean it up (and change some of the hard-coded parameters to proper settings and such!)