Well, since the topic has been resurrected, I'll throw in my 2 cents.
Just create a phpBB forum on a subdomain, like "forum.<ServerName>.com" and add a button in the CMS to take you to the forum. As far as logging in goes, just find a phpBB mod that utilizes the same encryption method as the CMS/server uses and link the forum credentials to the server's credential database. Of course you will need to manually create pre-existing accounts as the forum DB will only have the credentials and not have rest of the profile data for pre-existing server accounts.
Of course separate accounts would be the quickest and easiest method. I've been drinking so I'd rather not speculate on being able to write a script to automatically create forum accounts from the server's accounts database, but at the risk of quite possibly being wrong, I'd like the think you could since you should have the information for the salts.
