I was surprised how easy it was. In seconds bbpress was installed and sharing the same user table as wpmu.
I never could get wpmu to run on top of bbpress. Evertime I tried wpmu would think I wanted to create a blog named boards instead of load the installation of bbpress in the directory called boards. (If anyone has a solution for that PLEASE point me in the right direction) So I “faked” it. I included the css file from my wpmu theme. Made some static changes to the header of bbpress just using HTML and there you have it!