I Know a community portal link is given, however, I'd like to keep a phpBB at the side of the mediawiki.
Is this foolish?
Well, given that the users of the English Wikipedia have consistently failed to agree what mix of namespaces, separate wikis, mailing lists, and possible forums to use for different types of discussion, I think it seems a fair enough experiment. Indeed, it will be interesting for other wiki communities to see how well the two approaches can be combined, from a community point of view.
Has anyone tried using one user table for mediawiki & phpBB?
I'm very sure that it's possible, you might have to change some column names and make sure that they only share the user table.
Note in particular that there is recently-added (and possibly still "experimental") support in MediaWiki for database tables to have prefixes. I don't know if you could build on this to ensure that the user table has a matching name, but the others don't.
The main thing that you'll have to modify, I'm guessing, is account-creation routines: you want creating an account in one piece of software [i.e. MediaWiki or phpBB] to fill in the information for both pieces of software. Hopefully, passwords will be in the same format for both, otherwise you'll have to encode it twice. Otherwise, it will just be a case of find and replace on any column names you've had to change, I'm guessing.