They have a couple of ways to bridge phpBB-3 with ZenCart 1.3.8 now, although you lose the ability to register thru phpBB. New members register thru Zen, which subsequently creates their phpBB-3 account.
I would like to do this, but am stuck on the last piece (would be a snap if I could read php), where you replace one piece of code with another - I found the piece of code, I just don't know where the code ends, (i.e., if it's the > or ; or } what part!
At any rate, here is what I found (and btw, Zen does refer everyone back here - I agree w/Jim)....
http://www.zen-cart.com/forum/showpost. ... stcount=37
http://www.zen-cart.com/forum/showthrea ... 565&page=5