It works!!
So now I have 2 WP blogs (1 on the root and another in a sub-directory) and a phpBB installation in a sub-directory – and I can sign into all of them at the same time using your plugin!
I installed the plugin on both blogs and pointed to the correct paths for phpBB and voila!
Works perfectly. It even logs you out of all of them at the same time regardless of which page you are on when you click logout.
What a great plugin!!
If the next version includes some of the fixes that others have requested on here this plugin will become what a lot of people have been trying to find for a long time.