Hi all,
I've just the same issue and I'm searching for an answer.
With students, we developped a SSO system for WP & phpBB3 by modifying the OnePress WP plugin. But it's not yet perfect as so many things have to be changed in both softs. So we're still searching for a simpler solution.
If possible, we would have liked an app-independant system, but as we've fond none, the base idea for now would be to use the phpBB as master application and propagate the users to WP and MediaWiki. OnePress uses that and we based on it for our system.
However, thanks to Dog Cow, I've just discovered that a new version of WP-United is out on "12 June 2009" ! It's "v0.7.1 Release Candidate 1" and has been tested with "WordPress 2.5 — 2.8 & phpBB 3.0 latest". This might be a good path to explore. It's at
http://www.wp-united.com/get-wp-united/.