Re: MediaWiki, bbPress, and WordPress integration..
On my setup, (different system, same concept),
i used a centralized mysql table which contains a mesh of all of the variables of all systems!. The reason wy i did that is, some of these usertables contain information which would change the bahaviour of the software, such as roll, last login etc. SO its not a mater of authentication and session ONLY. You also need the other stuff!!!.
i ALSO used a single portal for signup/signin. THIS SHOULD NOT BE SCARY (assuming that thinking does’nt hurt your brain). A good IDE will get everything done for you (search replace globally, find all instances of a function etc).
I reccomend http://www.eclipse.org/downloads/ as an editor, and use PHP IDE as an ide ontop of eclipse. Be patiant, if you have’nt installed before, it could be a bit strang, but do take a better part of a weekend to figure out hw it works. TRUST ME, php will be your bizatch afterwards 
