I figured a way to create per-organic group specified forums, with no extra modules needed*. You will, however, have to apply this patch. It gives Views more respect for the $group placeholder. I am not using the core Forum module, but would be interested to see if this worked with it.
I developed this because I was worried about my users understanding the OG Taxonomy or OG forum modules. With this, they only see a few extra text fields when defining their group, and a pull-down when creating their topics.