No offense to those that want this bridge, but I would not recommending such a bridge with osC. Both phpBB and osC are very popular open source projects, and with that being said, this makes them very popular among hackers who trys to hack the core and files and getting your user info.
Now if your phpBB by chance gets hacked, there is little that you can do to stop the hacker from getting the user info from the osC side of your site and vice versa. At least if your site got hacked on the forum side and your users did not use the same login info on the osC side there is that little extra protection to keep all your customers finical info safe.
Do not get me wrong I am running both osC and phpBB well as J! 1.5 all in one site with phpBB and J! 1.5 bridge and osC stand alone all with their own databases, but for the very reason I stated above I keep osC separate from the rest of my site.
Also another reason why I would keep osC separate is because even though it is a create tool for e-commerce sites, the code is outdated, even with the new release of osC 3 which was just a rush job leaving old and outdated modules and security holes in it. I have mines stripped down to basically the bare bones and then once I patched all the security holes I went and one by one added mods and tested them to make sure all my patches holds true.
To me it just is not worth it, playing with my customers info like that.
Just my two cents.